At the end of March we visited Bicton Park (https://www.bictongardens.co.uk/) in Devon, South West England. Although it's billed as a Botanical Gardens it's also a very good place for children because in addition to handsome landscaped gardens and a countryside museum there is also a miniature woodland railway, indoor and outdoor play areas, ducks and swans, a cafe and a gift shop. We had a long and very enjoyable day there and would happily recommend it to anybody. Bit of a long commute if you happen to live in America but at less than £10 for adults and £8 for children it's very good value for money if you exclude the travel costs.
This is a photo of the ball pool in the indoor play area. Can you see anything else in there?