Hi Captainpig. No, we've not been to Sandy Balls. We had a run out to the Chesil beach site late year just to take a look. I would say it's more for older people as there isn't very much there. It is right on the beach though whereas the Waterside site is about a mile and a half walk away. It's not a bad walk though, I walk into Weymouth everyday while DP takes the bus with DD, I love it, it's all flat and you can either walk on the beach (which is more pebbly before you actually reach Weymouth front) or sick to the path. Or, you can walk part the way and then catch the little train that takes you into the town, it's just a little shuttle train that travels along the pavement.
Chesil isn't as big a site as Waterside and there doesn't appear to be as much to do. TBH, I found it a bit grim compared to Waterside, it's one of those places I feel I would go alone to, for quiet time, to reflect and walk along the beach. That's how I can best describe it.
No, Waterside isn't a loud arcade type place. There is a little fun park for the children in which the rides are very reasonably priced. They do happy hour type things, I think the rides worked out around 40p each last year. My little girl is six now and she loves it! All the entertainment is far enough away from the caravans so you aren't disturbed by any noise.
There's also a little beach area just a short walk away from the site, it's not a sandy beach, more pebbly but quite a few people tend to gather there. The adults can sit while the kids paddle as they are never too far away from you, it's lovely. There's a little pier too. There's also a very reasonably priced cafe where the food is good right at the side of that little beach.
If I were you I would look online at the site plan, the caravans are in sections indicated by colours. We were in the red area lasy year which is quite high up and up a little bit of a hill. We liked it's position and have booked the same area this year but you might like something closer to the 'action' so to speak if you are planning on taking your DD out in the evening, it might be a long walk (or a long way to carry her ) when she is tired, as I learned to my cost!