Agree with ejsmith's recomendations.
The park in Park Road is about 15 min walk from Wroxham centre and worth a visit with the little ones. There is a park heading the other way out of Wroxham, but the Park Road one is newer and a lot more equipment. Perfect for your age children.
Wroxham Barns is worth a visit. Won't be a whole day but a good 3/4 hours can be spent there. The petting area has a small soft play area included and is well enjoyed by my 2 DC. Tip - pick up one of the tourist brochures for Norfolk "Enjoy the Broads" as it used to contain a voucher for a free bag of animal feed (you want the large animal food, not the chicken feed)
Ranworth has a Nature Reserve which is worth walking out to. It's a floating visitor centre and the mezzanine level has bird watching windows, with binoculars, and an area for children to draw and complete a big jigsaw. The cafe on the staithe at Ranworth also does some lovely large slabs of cake, but if you are concerned about being near the water, the church up the road also has a nice cafe. You can also walk up the Church Tower, with amazing views at the top.
We've not really eaten out a lot in Wroxham as we have a fussy eater but most of the pubs are good, and likewise the Chinese and Star of India takeaways near the bridge. If you have some real ale drinkers in your party, can I suggest a visit to The Shed? It is literally a shed, and hard to locate but phone them and they guide you in! They don't do food but are welcome to order a takeaway and eat it there. They will even supply cutlery and warmed plates . Extremely friendly in there, and plenty of ale.
If visiting Norwich, the Park & Ride at Sprowston is the best value for money. Plenty to do in the city, shopping or otherwise.
Whitlingham Country Park is excellent too, just be aware of visiting at a time that Norwich City FC are playing. The roads nearby are used as free parking and you may be delayed by the amount of traffic.
If you decide to hire a day boat, download a £10 off voucher for Broads Tours. Has to be taken early in the day but with children, I doubt that will be a problem.
We've also gone further North and had some lovely days out on the North Norfolk Poppy Line. Caught the train from Holt (nowhere near the town centre btw) to Sheringham, had picnic lunch on the beach and a wander around the town then train back. You can't beat a steam train ride alongside a poppy field with the sea in the background.