We do the same Goodwill thing at our club every year. As a rule, the kids go in the back, with two adults (usually two pilots) up front.
It's not just the grabbing of the controls, but also things like panicking, vomiting etc., which you really don't want to have to handle on your own. From experience I can tell you that even a vomiting adult in the RHS (who brought his own sickbag and otherwise behaved impeccably) is a major distraction while trying to land.
Oh, and if you have a half-decent audio selector box, make sure you know how the "pilot isolation" and "crew isolation" buttons work. Yes, we brief the kids to shhhh while taking off and landing, but most of them forget as soon as they hear their own voice over the intercom.