It's going to be really hard (or probably impossible) to pick up a cheap deal at such short notice. The school holidays have already started in almost every country in continental Europe and the last week or two of the summer term here are the most popular periods, outside of the school holidays, for UK families to take their holidays.
If I wanted to take a youngster abroad, at this time of year and with very short notice, I'd forget about places like Spain or Greece. Instead I'd head for Ostend. It's a great place for families with lots to do and see.
It's unlikely that you'll find a cheap package to Ostend but, if you live near London, you can get there fairly cheaply on Eurostar. I've just checked some prices, going out on Tuesday 18 July and returning on Tuesday 25 July. (I deliberately chose mid-week, as prices are often cheaper). The total return price, ex-Waterloo, for both of you would be �119. (Eurostar tickets include free on-bound travel to any station in Belgium).
A quick websearch has shown that a twin room in Ostend, for 7 nights, might come to about �350. You'd have to add on meals but Ostend must have a greater density of eating places (mainly cheap) than any other place in Europe!
Ostend has got a great beach and there are plenty of activities specifically for youngsters. (Every Wednesday is Children's Day with different activities throughout the town. Plus there are the usual play centres with climbing areas, ball pools, etc). More importantly, though, there a lots of things for parents and childen to share. (e.g. boat trips or a tram ride to Blankenberge where there's a great outdoor family area with things like crazy golf and, even dafter, crazy snooker!). The tourist information office will have plenty of suggestions, including information about how you can get to several theme parks.
I've been to Ostend with youngsters several times and they've always