Hmm… I’m a kid who likes cake deco myself, so I’m just thinking what I would find easy,
A firm delicious cake that isn’t too crumbly (Carving a sponge/butter cake can turn into a nightmare without proper practice!)
Simple icing, not too sticky, crumb coating is of upmost importance.
If you like fondant, maybe a set of cupcakes (four to six cupcakes each child) and teaching them to pipe buttercream swirls and make simple fondant deco (like with playdough) would be less challenging and mind-boggling, yet TONs of fun. Cake covering and icing has to be a careful, thought-out process, and children are more often than not impulsive (can’t wait to get “into” the cake) and can overdo everything !
Just my thoughts, hope it helps!