You can mandate the chore portion of it, but the difficulty is mandating interest.
I would have a frank talk with mine and find out if they truly want the pigs, have any thought of them as beloved pets rather than just a novelty, aNd go from there. I don't take rehoming a pet lightly, nor do I think children should be permitted a rotating cast of characters in whom they quickly lose interest. But I would also not be willing to make the guinea pigs my life's work when they could have a happy home with kids who would love them.
And we would not be getting any animals until they were old enough to follow through.