Ooh Knodishall is just down the road from me! 
Definitely go to Southwold. The pier is fab and the beach is gorgeous, although a couple of bits are fenced off because it took a battering in the storms last year. It's got a lovely town centre and a really nice playground. If you've got a map, take the common road down past the golf club and where the sign is for the tennis courts, there's a lovely little cafe called Pit Stop. It's very much sandwiches and cakes type stuff but they've got a ton of kids toys and bikes there. We go with friends all the time, it's lovely. You can park in there.
Easton Farm Park is lovely. Lots of animals to see, soft play, ride on tractors, tractor rides, pony rides, lovely for little ones.
If you get a chance visit Snape Maltings. Not really for kids but gorgeous shops and cafe. Fab toy shop!
Minsmere Nature Reserve, as featured on Springwatch
, is fabulous for walks and they've got a kids playground.
Leiston has got an indoor swimming pool.
If you go a bit further down the A12 to Martlesham there's a big soft play place, if you like that sort of thing.
Africa Alive is a lovely zoo, and it is affiliated with the Dinosaur place and Banham Zoo near Diss.
If I think of any more I'll come back.