Rushmere is in Heath and Reach, near Leighton Buzzard. They have lots of bits there that are perfect for families including a toddler park with slide, little play house and other bits and park for older children with a giants slide and swing.
There is a large meadow opposite (but far enough away from) the car park. It is great for having a picnic and they have lots of picnic tables there. As is a large open space there are always kids running about, doing kart wheels (this is mainly what my daughter ends up doing!) and playing. If the weather is good it would be lovely to spend a bit of time here.
There is a trail that begins to the right of the meadow and goes all the way round, finishing back on the other side. This trail is a good distance for kids and has lots of wooden sculptures along it including fairy doors (which the kids love looking out for), a giants chair and giant spider. Children are welcome to climb all over them (which they obviously do :)) and there are also some great trees for climbing.
They have a small visitor centre and café and parking is £3.50, pay on exit.