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.