Rhiannon, I went this year with a 7 and nearly three year old and they really enjoyed themselves.
The best thing for them was the water maze - lots of stepping stones around an island wiht a tower you can climb. Some of the stones, when stepped on, activate small fountains which you are meant to dodge - try telling that to a child. But since you are going at half term, getting wet outside would be chilly so you'd need a change of warm clothes and possibly a towel.
The adventure playground has enough to occupy children for up to an hour I'd say. It's good but not big. Ornamental ponds are a diversion, too. Lots of intersting fish in them.
The hedge maze is fun, too, especially for your older child. The castle tour is interesting and only takes about half an hour. Not very four year old friendly, but what historic tour is?
The cafe sells lovely home made cakes ( one of the best selections I've seen) or did when we visited.
There was also a good plant shop, with lots of interesting and unusual plants on sale.
We would go back, but I think I'd pick a warm day as the main attraction for my two is the water maze.
I don't think there's enough to entertain a child for a full day, but certainly enough for a long afternoon. Also, if you're feeling especially rich and active, you could take in the Enchanted Forest at Groombridge Place, which is only about 15 mins drive away.