my brother could never talk. just make noises. But you could tell when he was happy or frustrated or upset. And we always had really good interactions.
And working with the characters as a photographer, many times there's no talking needed to have a great time. If they do feel the need to talk about something, make sure you're close enough for mom translation. But it helps alot if the kid has something in his hands. Like my brother loves balls and playing catch, so if he had a ball in his hands, the character could take it and toss it back to him. Or if his favorite thing is dinosaurs, wear dino shirts. That's where most the converstations start from.
And nine times out of ten, the little princesses get so star stuck anyways when they finally meet the princesses, there's hardly a two way converstation going on. It's usually the little boys that have the best converstaions with them because they don't aww the princesses.
And watch the attendant and photographer to help you out with character translations... I always did my best to know what they wanted from the kids. So everything went more smoothly.
And as annoying as it sounds, some characters do not give in the extra time they should with special needs kids. But don't just go up and yell for the whole world to know that "he's got XXX". It was always annoying to have parents just yell my kid has autism.
The best thing is to walk up to the character and introduce your son. Saying "Hi mickey, this is my son. Sometimes he has a hard time talking". And always allow for the character to control how much time they spend together. It's so frustrating watching little ones with the time of thier life being yanked away when they're still playing. I always tell the parents, if mickey wants to play, let him, he'll tell you when he's ready to say good-bye.