You're not remotely unreasonable - not sure whats going on in the minds of the 4% who chose YABU!
Mobility aids and how you use them seem to have a very strong influence over peoples view of you, at least I have found this to be the case.
Using a mobility scooter - scowls, mutters, dirty looks - judgement that I am using it because I am too fat and lazy to walk.
Manual chair, active style, self propelled - smiles, offers of help, but also pretending not to see me - like the time a castor tyre came off and I was sat on the pavement trying to fix it, on the verge of tears as I did not have a way of fixing it and my phone was dead. Everyone everyone walking past, looked away. (My DP eventually came looking when I failed to meet him where we'd agreed).
Manual chair - pushed by someone else - lots of talking to the pusher not me, offers of prayer, disgusting comments about my size hissed or shouted at me...
Powerchair - I am now invisible, I've been asked where my responsible adult is, assumed I am numb from the neck down (so its fine for their bicycle to smash repeatedly into my chair, jolting me)... fat related abuse again. People also look REALLY hard for someone else to speak to, resulting in folk behind me in a queue being asked stuff, and having to explain they are total strangers and not with me!
I've also had kids think my powerchair is cool (yes, yes it fucking is), tell their parents they want one (haha, squirm time!)... ask questions without bias or judgement, just curiosity.
Honestly the thing that upsets me most is when children are yanked out of my way (often when they were never in my way) and hissed at or told off for simply existing within a 10ft radius - because the parent is embarrassed that their child may have stared or got in the way or might ask a question.
I don't want kids growing up thinking wheelchair users or anyone with a disability or obvious (or less obvious) difference is something to avoid and fear - but that IS what these parents reactions will teach them long term!
So, answers to questions kids might ask:
'Some peoples bodies don't work the way yours does, so they have to do things a bit differently'
'Some people find standing/walking very difficult or painful, the chair helps them do stuff'.
'Lets look up some stuff about wheelchairs and disability when we get home, as that person is busy right now'.
'It would be rude to ask as it's personal and private, but we can find out more from the internet/books when we get home'.
And if they have already dived in and asked, just apologise a normal amount as you would when kids do impulsive and possibly, accidentally, rude stuff to or in front of anyone else, and have a chat with your kid later on.
A few dont's though....
Don't sarcastically send your kid to ask because you're fed up of the questions, hoping I will educate your child. I might, if I feel that way out but I might be in a lot of pain or on some pretty hefty drugs. I can't promise my answer will be kid safe, age appriopriate or sensible. If you don't want your child told that a shark chewed my legs off and these legs are just fakes the fairies brought me to put my trousers on, so DON'T JUMP IN PUDDLES LIKE I DID.... Or just told to sod off.... don't do it!
Don't yank your kid away hissing 'DON'T STARE, GET OUT OF THE WAY'... because apart from making your kid really not want to look at, engage with or acknowledge us cripples, it also makes me feel like fucking shit, that I am the cause of your kids embarrassment or upset, that I am something to be avoided. Sounds over sensitive but on a trip round a supermarket this will happen 20 times or more on a busy day!
Don't - let your kid run under my wheels, just watch where they are. 200kg of wheelchair, plus me, will do some severe damage even at 2mph.
Don't tell your kid to stand outside the accessible (and outward opening) toilet door to wait for you (I heard the instruction!), because the only way I have to open it is to kick it hard and if your small child is standing there, hasn't heard the lock disengage, hasn't moved... even when I shout 'coming ooooout'... welll. That did end in tears unfortunately, mostly through shock though.