Wanna make a scribbly mess of your bathroom? OK, put it this way, DO YOUR KIDS WANT TO?
Oh, yes.
Really excellent bathtime fun. Feeling tired at the end of a hard day? Pop your kids in the bath, give them a crayon apiece, and they will stay in twice the normal time, allowing you to have a hot cuppa and say things like, "don't draw on his forehead" and "who can draw the best duck on the wall?"
It's really cool if directed well - they draw lovely pictures, have endless fun, learn to write and get clean at the same time.
Just don't leave them alone with them - or you'll be cleaning it off for weeks. But then, you shouldn't be leaving them alone in the bath anyway.
Yes, you get to do some wiping after. Yes, your grout might be blue and yellow for a little while longer. But hey, you got the kids, you make the fun.
The crayons extend and retract which is all very well but they do break and you do have to fish them out of the plughole / ears / toy ships and try to squidge them back together again. It's good dexterity training for you though, so go with the fun of it.
On balance: BRILL. I'm on my 2nd pack, and I'll be buying more. Squidge, plop, wipe wipe wipe. ;)