Another option would be to go to the Haven camp Hafan Y Mor in North Wales.
http://www.havenholidays.com/Parks/wales/hafan ymor/
I went there a couple of years ago with some children and there was PLENTY for them to do.
This is a large camp that has a HUGE swimming complex with flumes etc, plus other plays areas.
There is entertainment EVERY night and the kids will find there is plenty to do while they are there.
Most of the entertainment is included in the price so you do not need to pay extra for the swimming or the evening entertainment.
The do half board so you could have breakfast and evening meal included.
There are some caravans but also self contained chalets so you can take yourself (you can also stay in a self contained chalet AND have the half board meals).
Accomodation details here
http://www.havenholidays.com/Parks/wales/hafan ymor/accommodation/
It is also in a lovely area, with Snowdon nearby, and you are on the Llyn Peninsula, which is a beautiful part of North Wales.
http://www.llyn-wales.co.uk/
If that camp does not suit you there are plenty of other Haven camps which would keep 5 young kids amused for a holiday.