Mary -
You don't necessarily have to have a reservation for the character meals. While it does help and it wouldn't hurt to have one, it's not mandatory at DLR. You will still get seated. You may have to wait a while, but you will get a table. My friends and I have gone to both Goofy's Kitchen and the Critter Breakfast with reservations and without, and it was fine no matter which way we did it.
The restaurants where you really need or should have reservations are Blue Bayou, as previously mentioned, and, in my opinion, Carnation Cafe because the seating is so limited there and the tables fill up fast.
Now, as for which character meal is best for your 4-year-old son....You will get many opinions on this topic, as it is very subjective. All of the meals have their pros and cons, all of the restaurants have their 'off days,' and you will find some people who say that one character meal is awful, terrible, they'll never go back, etc., while other people will say that the same exact meal is their absolute favorite and they will always go back!

So how do you choose? There's no right or wrong opinion.
Basically it boils down to this:
1.
Ariel's Grotto's breakfast and lunch in DCA is all Princesses, so I would imagine that your 4-year old is not interested in that?
2.
Surf's Up with Mickey's breakfast at the Paradise Pier is gaining a lot of steam in the character meal realm. This meal is quickly becoming a new favorite. This is the only character meal in DLR where you will find Mickey. You can also sometimes find Daisy Duck and usually Stitch, along with Minnie, Pluto, etc. Usually, there are around 4 or 5 characters there at a time, and the restaurant is not always that busy! The characters wear fun, beach-y garb and supposedly the actual character interaction with guests is excellent. The buffet has a few interesting items (at one point there were flautas - not sure if those are still there?), but it is not too large in general. Still, most people seem to really enjoy the food. This meal opens at 7:00 a.m.
3.
Minnie & Friends' breakfast in DL, hands down, has the most characters out of all the meals. So if it is a matter of seeing the most characters, this is the place to go. You may see anywhere from 8 - 14 characters, and they run the gamut from Minnie to the Pooh characters to Chip and Dale, the Mad Hatter, Captain Hook, etc., etc. Some folks say that the characters come too fast and furiously to be able to get a bite of food. Some say the food is not good. Others say it's great. It's a buffeteria-style set-up. This meal is actually a bit less expensive than the others, if I'm not mistaken.
4.
Goofy's Kitchen in the DLH (my personal favorite), along with Ariel's Grotto, is the most expensive of the character meals. It's considered "premium" dining. What you are essentially paying for is the size of the buffet, which is huge. It's the largest selection of food out of all the meals. There are lots of choices for even the pickiest of eaters. My friends and I love the food and have rarely had meals that were 'off,' but again, it's all a matter of opinion and others feel differently. The number characters usually ranges from 5-8. I usually see Goofy, Pluto, Baloo, Chip and Dale and a couple of Princesses, but Minnie appears here, as does Captain Hook, the Genie, Alice and the Mad Hatter, Aladdin, Honest John, Gideon, etc. This meal is open for breakfast/brunch (but not lunch)
and dinner. It opens at 7 a.m.
5.
Storytellers Cafe Critter Breakfast in the GCH is the only character meal that offers a (small) buffet or menu orders (which are not cheap). There are typically 4-6 characters, and they are mostly the "woodsy and jungly" Disney characters, including the ever-present Chip and Dale, Koda and Kenai, Terk, sometimes Meeko, sometimes Rafiki, sometimes a Brer character or two, etc. Some people love this meal because of its laid-back, calm environment (unlike the high octane fun of Goofy's or Minnie & Friends). For that same reason I also kind of found it dull and blah!

I guess I like high octane character fun at the character meals! People also rave about the food, but I find the food to really be no different than what I've had at Goofy's, honestly. It's buffet food, but there is less of it at the Critter Breakfast. On the menu you can order a full-siized Mickey waffle, but they will also bring out some mini-waffles if you ask for them. This meal also opens at 7 a.m.
My friends from out of town are visiting DLR later this year. They have a 4-year-old son and we are
definitely going to Goofy's,
possibly going to Surf's Up and
maybe Minnie & Friends. They don't want to overwhelm their son with so many characters right off the bat so that's why Minnie & Friends is only a maybe right now. They want to see how he handles just a few furry characters before introducing a bunch of them to him at one time. I've had enough of the Critter Breakfast to last me a while, so that's why we're not doing that one!