How about Princesa Yaiza in Lanzarote?
Here's a link:
http://www.totstoo.com/hotel/princesa-yaiza/
Your family of 4 could stay there on half board for about £4,500, including return flights and private transfers.
If any of your children are aged under 3 then it would be even less as then half board becomes complimentary.
Princesa Yaiza is brilliantly set up for children. There's an excellent kids club with playground and dedicated pools and a soft play area for under 3s. A special shout-out too for the popular ice cream parlour!
The luxurious setting makes it a lovely holiday for the parents too.
Have a look at the link for more info or call the Tots Too team on 020 7284 3344