I recommend Fishers Farm park near Horsham. They have accommodation though it is pricey, but there must be other options nearby. It's got rides, animals, climbing, trampolines etc. It kept my son happy for a whole day, and he wants to go back. And unlike any other 'theme park' ever, I would too! It's on the right scale without constant noise or big queues. To the east of Horsham is the llama park which we didn't get to but looks good for another day out.