Don't worry about it. A lot of (most?) places use the accessible stall as a stall towards their required-number-of-stalls count, so the stall is there for everybody to use (by the numbers). Obviously, if somebody who is obviously disabled is waiting for it and you don't need it, use a regular stall. Or, if you try to use it and somebody says they need it for invisible disabilities, of course. But, if you need it for whatever reason, use it and don't care about what others assume.
If your kid using a wheelchair can't be left alone or with somebody else, that's a valid reason. I don't need the higher toilet (hate it, actually) or rails of the accessible stall, but need to use it because I can't fit with my service dog in a regular stall. If I'm in a restaurant with family, sitting close to the bathroom, and choose to leave my dog with a family member (never with a friend or anybody else), I'll use a regular stall (this is an easier situation for me, but often not possible). Otherwise, I need to use the accessible stall. Obviously, I would never leave my service dog outside the stall alone, just as you would never leave a young child, especially if she has special needs, outside alone.
