Squeeeeeeeeeeeeeeeee piglets 
Do you know what they've been scoffing before you got them?
Off the top of my head (thinking what's in season and a favourite of my boys)
Sweetcorn (on the cob)
Carrot
Cucumber
Celery (ours love the leaves)
Rocket
Salad bags (but NO iceberg)
Peppers (mine like red/yellow/orange )
Romaine
Apple- tiny bits
Bananas
PARSLEY -GP1 favourite
Cauliflower stems (I give mine the white bit buy only a tiny bit)
Pasnip peelings
Turnip
Careful with cabbage, brocilli, greens- can give them wind.
When strawberries, grapes, melon are in season they love them
Offer them little bits. I give 3-4 choices for supper, different 3-4 for breakfast.
And if you can , fresh cut grass and dandelions.
There are loads of lists on threads here.
There are some definate NOs:
iceberg lettuce (too watery)
potato (any part)
beetroot leaves (the actual beetroot is fine but not in season)
Anything with a stone (peach, plums, cherry)
Lots of weeds they can't have,
Careful with herbs, mine go arsey with coriander. They love parsley but only as a treat.
And: anything you wouldn't eat yourself , you don't give the guineas.
Enjoy them, they're lovely.