WASD keys are not done for AZERTY keyboards. I (but i am stubborn) will not ever play a 'WASD only' game.
And i dont think there is a web standard for kids with disabilities.
Wich is rather nice, because up to you to invent it. The possibilitys are wide and wild.
I would go for keys that can be chosen, starting from a configuration that you think is fine.
Also, i dont know if that is enough for kids with disabilities. Those keys are really tinny, certainly on the latest cheap laptops, and we all know that kids with disabilities have a big chance to fall in the market target for those tinny laptops.
I would use space to select/deselect. Its a big key. But i would also experiment with a rather totally different approach.
Touching any key on the right of the key board > move right.
Touching any key on the left of the key board > move left.
Same for up and down
Maybe you should even reconsider if up and down are really needed. You could do the up/and down in a automated way. Move it right and it moves automatically (and smooth) up/down to the nearest target. Now you have a big space bar, and the whole left and right of the keyboard to do the stuff. Dont matter if they press several keys at once, because 1 key is for some really really difficult.
Anyway, taught i give you my 2 cents.