In reply to mo:
loop of elastic around the wrists or ankles to limit movement
clip the back loops of their climbing shoes together
blindfolds
let someone else pick where they have to move next
"add a move" take it in turns to go on the bouldering wall. first each person moves one hand, next person does person 1s move then adds one more of their own..continue..if you can't do the move, fall off, forget the sequence etc you are out (feet can go anywhere)
twister...great for a laugh, convince the wall to put up 12 or so holds in twister format then play as normal! i suggest you let the kids pay it hands only (feet anywhere) to start with! variation, both players move at same time
climb with a full rack - they may not need to use it but it will slow them down!
climb easy slabs feet only
climb with one hand/leg etc