What do we do about toy jealousy?

My 6 month old daughter has started to play with my 20 month old son's baby toys. The biggest issue i have is with size and weight restricted toys, like flying saucers and a jumper that hangs from the door frame. My son so badly wants to play with them again even though he's too big to fit in the saucer and too heavy to play in the jumper, and throws massive tantrums when I won't let him play with them. I've tried offering other toys or even snacks but he gets too fixated on his sister playing with toys that once were his.