I don't think you have a lot to lose at this point. Gosh, kids can make some great messes, huh? Is it thick, or really ground in? I might be inclined to see if you could chip it off with something lightly if it isn't on there real thin. Maybe like one of those textured dish cloths. My only other thought would be an oil of some kind like baby oil, or maybe even Goof Off. Goof Off removes that type of stuff, but I'm not sure how harsh it is. I think it is mainly oil based. The oil will soften it though, maybe to the point where it will come off. Used lightly this might be OK. If it isn't really soaked in, the water might do the trick though. It has to wear off eventually if you keep at it, right? When you get to the point where you really have nothing to loose, the only other thing that might work in a hypoallergenic, acetone free nail polish remover. Gosh, I feel for you. My husband would have an absolute fit, so I feel your pain. Good luck.