A few years ago for my sisters 10th birthday we put fairy liquid on the trampoline and then get the hose and made it wet and very soapy,its so much fun slipping around,may be a bit dangerous with tiny ones though!
Play "Dodge It"
Again put fairy liquid on the trampoline
Put a ball in the centre and the players circled around it
Everyone starts bouncing and have to doge the ball but not slip over.
Whenever the ball is about to fall off everyone has to sit down as quickly as they can and whoever is the last person to sit down has to get the ball.
Then it starts again!!!!
Have fun!!!!!!!!!!!!!!!!!!!!!! =D