Although it is free I'll pre warn you... as you enter you have to go through a till where they persuade/coerce/guilt trip you in to giving a donation! Think the recommended donation last time was £3 per person, but that was a while ago. They also charge you a fortune to park!
Saying that though, I'm sure Holly and James will enjoy it and even at £3 per person it's still pretty good value. There's a little minature train, that you can pay extra to ride on. And there's an outdoor and little indoor play area bit.