Our daughter is 6 now, we've been to all of the characters meals at least once and have done BBB 3 times. Opinions on each follow, but it is really all based on your granddaughters preferences and favorite characters.
BBB - we've done the castle package every time. She loves the idea behind it and all the attention at the salon, but we end up spending the day convincing her to keep the hair-do as she always wants to take it down within about 30 minutes because it either itches or hurts! Oh, and she always wears a camisole or white undershirt and shorts under the dresses. Given all of this, if it were her first time, I'd still definitely recommend it. It's an experience that can't be replicated anywhere else and she will definitely feel like royalty!
1900 Park Fare -- much different experience if you go for breakfast vs. dinner. We really enjoy dinner more with Cinderella & Prince Charming. Lady Tremaine and the step sisters are hilarious. Food is decent, service is ok, picture opp before your meal (offered to buy while you eat).
Akershus -- this is our favorite character meal (lunch or dinner), still! She loves the picture with Belle and the table visits with the other princesses, and the princess promenade through the restaurant. The food here is not the standard disney buffet, so do check the menu. Many people don't care for the Norwegian cuisine...we love it! The cold bar with various cold cuts, smoked salmon, pickled herring, salads and fruits is a nice starter to the menu items. Yummy desserts, too. We've always had good service here, but usually talk to the CMs about Norway and the food. They appreciate that you are there for more than just a princess meal. We went there before it was the storybook dining...it's definitely a favorite for us.
Cape May Cafe -- nice breakfast, doesn't seem like as much variety as many of the other character buffets. The characters seem to spend more time at your table here and it's not as rushed. But, it's a little harder to get to (unless you are staying at YC/BC/BW)...good for an Epcot day though. Our DD is indifferent about going here. Quick service in our experiences.
Chef Mickey's -- definitely a must for the Mickey & Pals gang. Cute picture opp before you eat (for you to buy while you eat). Nice interaction with the characters, although I always feel a little rushed. The buffet has a nice variety...we've done BF and dinner and they are equally good. We usually do this on a Magic Kingdom day since you can hop on the monorail and go to the Contemporary to eat and then walk or monorail back to the Magic Kingdom. Service is hit or miss.
Cinderella's Royal Table -- it's in the castle...need I say more! It's nice that it isn't a buffet, but sometimes the food comes out so fast I wonder if there's not a buffet in the back! It is a nice picture opp with Cinderella before you eat, but the princesses fly by the tables during your meal. There is a little ceremony during the meal that the kids enjoy, but it's all very quick. None of the character meals are really meant for true foodies, but this is probably one of best character meals. It's just a magical thing for the little girls to eat IN the castle...great if you can get a window seat and look out over the park while you eat. Service is almost too quick! You might get your check before you even get dessert, LOL.
Crystal Palace -- can't get our DD to go back there, she is way over the Pooh thing and the food is not enticing enough to get us back there. I know lots of people really like it, but it is on the bottom of our character dining list. There is a decent variety on the buffets, service is not usually very good though. Took us 30 minutes to get our bill once, ugh!
Garden Grill -- sorta cool to be rotating around the land...freaks our DD out a little though. We've only eaten here once and we had one of the last ADRs. LOTS of time with the characters...like they sat down with our DD and "talked" & played with her twice! She enjoyed that! The service was very attentive and the food was served, which was nice, but not anything to rave about. DD hasn't asked to go back, though she did enjoy the one time we went.
Hollywood & Vine -- our DD will tell you this is for babies now! But, she LOVED it when she was younger. Glad that she has grown out of it since the buffet is mediocre and the service not any better. If your granddaughter is still into Playhouse characters, then consider, otherwise, just do the parades and enjoy the park and eat elsewhere. 50's Primetime is ok, but our daughter just wants to go for the milkshakes. DHS is not the best place for dining.
Ohana -- only been there once for breakfast since our DD is not fond of Stitch! Not a large food selection, not one of our favorites. If she really likes Lilo & Stitch, then consider.
Tusker House -- We love this breakfast! Nice selection with some non-standard fare. Character interaction is a little rushed, but they do a cute parade around with the kids. The room seems more noisy than other meals, but this is a great way to start your AK day. I do like that the buffet is in a different room. Looking forward to going back in a few months! Oh, and while at AK, we never miss a few safari trips (if lines allow), DD loves it. And, of course, the ride over to affection section at Rafiki's Planet Watch. This is actually the #1 thing on our DD's list, but she is an animal lover.
Other ideas-- our DD loved eating at Cali Grill and being able to see the castle view. The kids sushi dessert is adorable and she loves it.
Our DD is an autograph collector. On the last trip we bought a canvas Disney bag that is mostly an off-white color. We carried the bag and a sharpie marker (bring two from home) everywhere and had the characters autograph the bag. 30+ signatures later, it was a lot of fun looking for characters that you don't always see out so frequently. The characters really enjoyed singing something other than another autograph book, too. And, it' s a nice souvenir. Next time she wants to do a shirt, but I am trying to think of something that she won't outgrow. You can check the
Disney store online for specials on items to buy ahead and take with you. We have bought some things (like PJs, gowns or tshirts) and packed, then give them to her as a surprise in the room before bed or heading to a park. We've also bought things at the parks or DTD without her knowing and had them sent to our room (if you are staying at a Disney resort) to bring home and use as xmas or bday gifts.
You can get free pixie dust sprinkled on her hair at the Barber shop on Main Street...in case you don't get enough at BBB!
If you are in a store that doesn't seem too busy and there is a CM available, ask for the special phone call and they can arrange a character to "call" your granddaughter. If it's not a special occasion, then maybe to just wish her a great day.
Our DD enjoyed Teppan Edo, but for some reason, the room was not drafting well and smoke/steam was getting caught in the room...so she coughed through the entire dinner. I am sure this was a rare thing, but we haven't been back. She enjoyed the food though and told me she wants to go back there again.
Ok, that was a long post. Hope it helps some. How exciting to be going with your granddaughter for her first trip! No matter where you eat, it's just all so magical for the kids (and even us adults)...as you already know.
(27 trips and counting 
)
She loves the princesses, loves Tinker Bell, loves watching me put on make-up and do my hair, loves when I do her hair, and her favorite color is pink...we all call her the "little diva" of the family! But at the same time...she loves to run around, play and is very adventurous! 
