Well i wouldn't do what TSR2 originally said and refuse to take your seats as this will just get you off on the wrong note with the cabin crew. As an sccm myself i always make sure families sit together even the ones who are the last to check-in. It is a CAA recommendation but as we all know recommendations dont mean much.
If you dont want to pay £10 each then just turn up 3/4hrs before and be the first to check in. Keeping kids entertained in the airport for that long is hard so i'd pay the money! have a good holiday!