We used to take our kids when they were young and they just loved being on the beach all day. There are 3 main beaches which are all different so it's a bit of a change to go to different ones, though we used to hire a beach chalet on Porthgwidden for the fortnight as the grandparents were with us and I can say that all 3 generations loved every minute there, and you can leave buckets and spades, inflatables etc so you don't have to carry them about all the time.
We still go but the kids are grown up now and the grandparents no longer with us so we go for ourselves. Some friends recently bought wet suits for the kids and small surf boards and they spent most of the time in the water as the wet suits kept them warm.
We love eating at the Porthgwidden or Porthminster cafes. The food is great and if the weather is good you can eat outside overlooking the beach. Love doing breakfast there. There's one at Porthmeor too but haven't tried that one. At lunchtime we tend to go early to Porthgwidden to try to get the window table with great views, hope you're not going the same time as us!!
There are loads of places to eat around the harbour and we've had great evening meals at Slipways restaurant on Fish Street. Hope you enjoy St Ives, we love it even though it's very busy.