We went on the Saturday of Easter weekend and although it was busy it didn't make it unpleasant. It opens at 10. I was thinking of going back again tommorrow too.
We arrived at 12.30ish and waited until late afternoon before going on the rides when most people had been on. The wait for the rides is nothing like the big theme parks though.
It's a theme park especially aimed at the under 10's and I saw big brothers and sisters having fun with the little ones and they we're complaining so sure your 6 year old will love it. There are pirate themes, cowboy theme and farm themes.
I have a 5year old Ds and 2 year old DD and plenty to do for both without going on rides. If weather is nice there is a good sized sandpit area with slides, lots of other play areas and discovery areas, rides, gocarts and picnic areas.
Lots of little play houses for the kids to play in. There is an indoor soft playcentre serving hot food and a snack bar serving take away food and snacks next to the under 5's playarea.
I have happy memories of going as a child 35 years ago when it was pets corner because of the little houses. It's £8 each to get in but although I initially had a moan it was worth it rather than paying per ride - we'd have spent a lot more at the coast!
Have fun!!! Let us know what you thought.