If you're running at a park and ignoring all the kids on the playground, don't stop to greet (or gawk at) a disabled child. If you're greeting all the kids at Sunday school, don't skip over them (or switch to baby-talk just for them).
So if you default to treating them like any other human, they'll be happy and you'll make their parents happy. And at this point, parents will happily let you know if there are any specific ways to interact with their child.
Note: Some kids have aural sensitivities ... don't yell at them like their deaf during your first interaction. I usually start with a voice slightly lower than my regular speaking volume.