Been to almost all these restaurants. Check out the link below for my reviews. Pictures are posted now and I'll be done the reviews in a couple more days.
In addition to TRex, I'm pretty sure Garden Grove is not on the dining plan either. I would pick restaurants that are convenient to your resort and park planss, I wouldn't make too many special trips just to dine (waste of travel time). Although, we will make some special trips like from Magic Kingdom to Wilderness Lodge because the boat ride there is enjoyable too.
You said you are set on character meals, but I have a few thoughts anyway. Are you going to Cape May Cafe for breakfast? We thought the breakfast was a nice buffet. Never been to dinner, the menu didn't appeal to us. Garden Grill is now one of our favorites. Are you going to Hollywood Studios? We liked the character breakfast there - DS at age 3, 4, and 5 liked going there and dancing with the cast and einstein characters, but was very happy this recent trip now that Agent Oso and Handy Manny are there. Lunch food isn't the greatest, but not bad. No characters are dinner.
Whispering Canyon Cafe is fun, the jokes are a little corny... but I didn't think much of the food at all. I personally would not go there again (been there twice already) unless I was staying at Wilderness Lodge, just not worth a special trip.
Coral Reef was great! Had a table right by the window of the aquarium and it was fun to watch the fish, sea turtles, etc.. I'd ask for one of those tables when you get there...
So.... when going to Garden Grill, be sure to do the Living with the Land attraction just before your dinner. And before San Angel Inn, take the 3 Caballeros boat ride in Mexico.
I don't like Ohana's dinner at all. Been there twice. The menu really has nothing for young children in my opinion... the wait just to check in can be awful at times, and the wait to be seated long. The food is not that great... (except for dessert), it's yuck. This is one of those places that get a lot of hype because it's all you can eat. The breakfast at Ohana with Lilo, Stitch, Mickey, and Pluto was fun. Went there twice in 2009, I'd do that again. And while at the Polynesian, you can get free fabric leis at the check-in desk.
Frankly, Le Cellier is another one that gets a bunch of undeserved hype. But at least the food here was good, unlike Ohana. But it's nothing to get excited about like the reviews would have you believe. And I think it's boring for young children when there are better places for families with young children to eat.
So, here's my quick list of places I think are fun for young boys based on my experience with my own DS at age 3, 4, and 5.
Hollywood Studios -
Hollywood and Vine Breakfast - characters and dancing
50s Prime Time - great food, fun theme, love the milkshakes.
SciFi Diner - food OK, but DS likes sitting in the cars and watching movie clips
Animal Kingdom -
Yak and Yeti - love (i mean love) the food, great service, flexible menu for kids, fun decor to look at.
Magic Kingdom -
Crystal Palace. You already picked a winner.
Epcot -
Coral Reef - fun to watch the fish
Biergarten - German music and a dance floor, decent buffet with variety.
Marrakesh - interesting music and belly dancing, flexible with the kids menu to provide other foods.
There are SO many restaurants to choose from! 
I wanted to get some input on some great TS restaurants for very young children, boys aged 2 & 4, not character meals though. I know about those. They must be on the DDP since we're going to be doing one meal at T-Rex because my son is obsessed with dinosaurs. 
You are right about T-Rex, it is not on the DDP. 
We just have to do it since my oldest son absolutely loves dinosaurs. I'm just praying that it doesn't terrify both of them! 
I'm so excited that he's getting excited about the trip too. I keep showing him different things that we'll be doing. I can't wait to go!!!!!! 
