Visiting any major attraction over the school holidays is asking for trouble, but there's no doubt that the Sandcastle was at full capacity over the Easter break when I visited. After getting over the shock of the price (over £30 for two adults, two children under 7 and an infant), we got off to a reasonable start with the family changing rooms, which made it much easier for us all to get changed. The larger lockers were in short supply and not really large enough, but we managed. Exiting the changing rooms, however, we were hit by a wall of noise and the sight of hundreds of people bobbing, splashing, yelling and generally having a marvelous time - it did have a touch of Armageddon about it though, purely due to the sheer numbers. It's not an exaggeration to say I could have walked from one end of the complex to the other without touching the water, had I borrowed the bobbing heads as stepping stones. Consequently, there were huge huge queues for the major slides . Since we had a baby, four year old and a six year old, this was a problem, as one adult had to keep two kids entertained while the other waited in line for the slides.
That said, there was a jovial atmosphere about the Sandcastle and no hassle with fellow swimmers despite the crowds. I felt that an exclusive baby pool would have been a good idea, as things in the "shimmering shallows" got a little hectic at times. You also need to remember to take the kids' water wings even if they can swim, as all three of ours were required to wear them since we had more than one child per parent. It's worth checking what the requirements are if you have more than one child per parent. The water temperature was good for the baby, who stayed remarkably cheerful for over 90 minutes in the water, without any signs of hypothermia.
Things went slightly downhill when we went to get changed, as the changing rooms were now at bursting point and we had to wait. Piles of used swim nappies were accumulating on the floor between changing rooms and staff, although helpful, weren't quite on top of the cleanliness. I would imagine that this was simply due to the peak season.
Overall, given the age of our kids, I think we would have had as much fun at a more basic swimming pool and would certainly have spent much less money. The Sandcastle is probably best value for ages 8+, and I would seriously consider selecting a none peak day if you intend to visit. If you can get there on a quiet day, kids of that age group will have a whale of a time.