Forgetting disabilities for a moment: if you are not used to being around _children_ then the first time you are surrounded by them you'll act completely weird. Because they are different than you're usual pattern. With sufficient exposure you adapt, you integrate that slight variation into the pattern and all is well. For some patterns you act differently (you act different for children than adults) for some you won't act differently (for most disabled children you act the same as for other children) but in all cases exposure is the key because we humans are imperfect pattern recognition machines.
Also though, some people are just assholes. I mean don't baby talk to anyone ever.
Which goes against your entire point, doesn't it?
> Some kids have aural sensitivities [...] I usually start with a voice slightly lower than my regular speaking volume.
OP says to treat people like you would anyone of that age. I take that to mean that he speaks to children in a slightly lower voice than he would older people, regardless of (perceived) disabilities or lack thereof.
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.