We've had non-sailing guests on our cat and found "Pooh School" was something that needed addressing. Easy enough rule, "if it didn't go in ya, it cannot go into the head."
3 young novice sailors had no problem with operating the facilities.
As far as cost sharing, someone suggested the food and drinks get split 4 or 6 ways depending on the number of guests and everyone tossed $50 or $100 into the kitty for things like ice and moorings, replenished as needed.
As posted, keep fingers out of winches, and if the Skipper needs help, he/she will ask. We found having the kids sitting on the port side louvers (we're on a Leopard 40) when anchoring or mooring had them close enough to see stuff, but out of the way of the skipper and anchoring team.

Don't sweat the small stuff, and remember, its usually all small stuff.