Just off J13 Newbury, northbound is The Living Rainforest, has playground outside, cafe, loos. Quite small place.
In Newbury there is Fantasy World, a soft play centre for all ages (up to 12) with food. Not beautiful for you though! But good rainy day option.
Bucklebury Farm Park between Newbury/Reading is very relaxing and in lovely countryside, animals, playground, indoor slide for the eldest 2 perhaps and of course animals. Not huge, but we've enjoyed visiting there. I just sit there and picnic with friends whilst they run around! You can seem them. There is also a tractor ride to see Deer every now and then.
Biggest and best place for you I reckon is probably Beale Park J12 M4, towards Pangbourne, alongside the River Thames, it has birds, animals, HUGE sandpits and paddling pools and playground and food etc etc!!
Or the Look Out Bracknell (think come off at J10), which is a hands on discovery centre, so they may be a bit young, but there is an outside (in the woods) playground, good place to let off steam.
I really want to take mine to Bowood House, on the M4 a couple of junctions after Newbury westbound, lovely gardens, but with an amazing childrens adventure playground.