We've stayed there quite a lot. Try to get a penthouse room as you get a nicer balcony with lovely sea views. We didn't find that the main restaurant was that good so ate in Sands Brasserie every night which was very nice. The beach there is amazing but will be busy at this time of year.
They have family rooms which are nice. You get a big bedroom with another room off it. You need to get a room above the restaurant as baby monitors don't work in all parts of the hotel. There is a play room with balls in and a nice indoor pool but no baby pool. The Haven Hotel just down the road is run by the same group of hotels and has a lovely outdoor pool with baby pool but no indoor play area. Sandbanks do a kids tea at 4ish which is quite good. When we went the food for children was nice and lots of fresh fruit etc. The main attraction at Sandbanks hotel is the fab beach.