It's well known that the way to get the best deals for last minute holidays for families, with or without the kids, is to book late!
Not only booking late [within 4 weeks of departure] but also not being too fussy with the destination.

Bets places to look are on teletext, the sunday papers and websites like bargain holidays .com. Once you have found a cheap deal ring, rather than book online - its possible to save money by haggling over the phone, and you'll often find different travel agents offering the same packages, and trying to price check the competition. Make sure that any unnecessary addtions [eg over-priced travel insurance] which you don't need have been removed from the package, make a few extra calls back and forth between the agents, and you could find that you have saved up to 50% compared to the peson sitting next to you on the plane.

Good Luck!