So the $4/kid is available? Charge an additional $5 in 'material fees' per kid.
Also, offer a 'keep your robot' option for an additional $5. What parent wouldn't pay $5 when the kid begs them to take it home?
Doing both of those, you can expand your budget to more like ~$12/kid . . .
Other options are to ask the kids to bring their own batteries, too.