While guinea pigs can eat pumpkin, it isn't something they should have a lot of.
I would try offering them a very small piece each to start with and see how they get on with it.
I have only ever offered a small piece of peeled pumpkin (no seeds) as I worry about what could be on the skin (but that might just be me being overly cautious of course).
Whenever I introduce a new food to my lot I always start with a piece about the size of the top half of my thumb.
Just start off with a small piece each, they may not like it or they may devour it! Pumpkin is high in fibre so expect lots of poops over the coming days. Sometimes people give pumpkin purée to sick guinea pigs during syringe feeding.