I can't give you a clear answer on the leaves, but I do know that the yellow squash themselves are A-OK for a bun to eat
See this Vegetable guide:
QUICK LIST: Rabbit-Safe Foods
Just be sure to introduce it very slowly, in small amounts to your buns. It may not agree with all bunny stomachs equally, so it's best to take it slow.
Take some cute pics of squash nomming, eh?