The whole leisure trade is unfortunately guilty of this practice. This year I took my 5 year old out of school a week early and saved £300 (50%) on a midweek Center Parcs break,
Virgin Holidays were offering free kids places for 2006 however when you look into it the offer is only valid for a few hotels at certain times.
One interesting point is that private schools have longer holidays than state schools so "bargains" can be got in the 1st and last weeks of holiday (not that I am so lucky). On an average £8-10k per year school fee the parents (single child) taking 2 holidays a year will recoup between £1500 and 2000 by having cheaper holidays.