A: An adult - nicely ask for some kind of refund. Paintjob and time spent painting not a big deal.
A child - hard to imagine myself in that situation, no children where I play. Probably wouldn't say anything, but keep a close eye on him next time.
B: A child - I would NEVER EVER let a child near my painting station. NEVER. Not happening. Brrr..
Hell, that' the reason why I refuse to have any cats or dogs inside, as they would most likely destroy everything.
An adult - Ask for refund.
C: Say I'm extremely sorry and offer them a refund.