They will be ok outside if it is warm but once the weather cools down they need to be kept indoors. You can get specific terrapin food from pet shops, but you can give them fish food (the floating sticks or pellets are ideal).
They need to be able to get in and out of the water as they like to bask in the sun or under heat.
Take care handling as they can nip, and they can grow to quite a size.
Hope you are able to take them on and give them a good home, as they are fascinating pets.