1) It has always been policy that ANYONE entering a character meal, buffet, or AYCTE pays the price for their age.
2) Once in a while, I have seen a restaurant manager skip the charge for kids with feeding tubes (or maybe a MAW family member).
3) But I have not heard of an exception for not wanting to eat by preference or medical condition (like autism).
. . . there is no way to keep track if they actually eat of not
. . . they would take time from the characters, so an entertainment fee needs to be charged
. . . the exception could not be evenly applied (HINT: Permanent employment for lawyers)
4) Be prepared to pay full price, for the age group, for the child.
NOTE1: As mentioned above, the kitchen could usually puree a menu item for you, although I would not personally recommend pureeing M&M's. However, for health-code reasons, they can't reheat (except baby formula) or puree food you bring, as the kitchen has no control over the purity.
NOTE2: Some say to call the eatery. This is no good, because you care calling the Cell Canter and not the eatery. The Call Center can transfer the call to the eatery, but the eatery has to FIRST agree to ace the call transfer, but only if the eatery agrees. And, the manager would not agree to such a request in-the-blind. Anyone could be claiming a disability on the phone. It is best to ask for the manager while at the check-in podium and hope the stars are aligned just right that day.