I think it all depends on your child. We don't stay away from signature dining just because our children are young.
Buffets are a toss up...it can be a blessing that the food is available right away especially if your little one is a good eater. But do you want to get up and down all night? To some it doesn't matter.....
Our girls are now 3 and 1 and the oldest has been going since she was just over a year and the baby since she was 4 weeks old.
Biergarten - I wasn't crazy about the food but the girls loved the show and the ability to get up on the floor and dance.
Chef Mickey's, Crystal Palace, Cinderella's Royal Table, Liberty Tree Tavern etc - the girls like the characters and it keeps them busy.
Concourse - the monorail going through also entertains them.
California Grill - luckily we've always had a great view of the MK and the fact that the tables clothes are paper the eldst has a blast coloring (as do we) and they always bring out a bowl of goldfish for an appetizer for the kids.
Ohana is always fun...views of the MK and my eldest loves the coconut races, hula hooping and the guy singing/playing music.
Whispering Canyon my eldest also enjoyed...the loudness, the antics and the pony races, etc.
Sci Fi - we liked a lot but our daughter is not in a high chair so all she wanted to do was climb from the front seat to the back over and over...
Le Celliar - they were fine - there's nothing to entertain them but yourself in there.
Rose and Crown - fine here as well especially if you get a water view seat.
Brown Derby - not much to entertain here but looking at the "photo's" on the walls are fun.
Those are some of the places we've been with the girls. They are used to eating out as well as their parents lingering over the meals. That's is why we don't shy away from signature dining, but you know your child and family best.
Good luck and have a great trip!