I think I view it as the same as marriage, there should be no reason why any disabled person cannot have the right to do whatever any able person can do (sorry if I get my terminology wrong, please correct me if I do)... but there has to be an understanding.
If any child wishes to make the sacrament, if they understand it, if it is something that they believe important to them, then they should not be disallowed.
However, I'm sure there are many many children who take their first communion without much serious thought, much as we'd like to think its sincere sometimes it isn't.
Each case should be dealt with individually. So I can't say whether YABU or not without probably knowing the child in question, its possible they understand and want first communion, in which case being disallowed is wrong, but there could also be a valid reason, that reason should apply to any child regardless of ability.