I came back to address the general issue of what to say to a person who uses a wheelchair. Or has a service dog. Or uses a cane or walker.
Try to say something that doesn't involve the disability or the special equipment. Look at the person--what would you say to them if they were able-bodied? Say that.
The OP and her sons were at a street fair. Appropriate comments might have been: "Have you tried the deep-fried Snickers bars yet? They're yummy!" or "Two blocks down, there's a bagpiper who has bagpipes that shoot fire! Don't miss him!" or "Are you staying for the fireworks tonight?"
If the kid has on apparel with a sports team's logo, feel free to ask, "How about them Sox?" Or if they play video games or what their favorite subject in school is or what's their favorite flavor of ice cream or if they think it will rain or are they hoping for a lot of snow this winter.
There are a gazillion conversation topics out there. To focus attention on just the person's disabilities--well, that makes them feel as if they are different. And they might be feeling a whole lot different anyway. Ignore the walker, the braces, the cane, the wheelchair, the dog. Assume, unless you find out otherwise, that the person is just an ordinary person who has likes and dislikes and favorite topics of conversation.
To you, the chair or the dog or the cane are new and interesting. To the person using them, they are very familiar. They are probably more interested in talking about other things.