Hi. I try to take advantage of as many "freebies" as possible ie the children's workshops offered by many museums and galleries during the holidays. I keep costs down further with picnic lunches. I usually give them a £1 or £2 to spend in the shop as a reward for good behaviour.
Ive budgeted £100 for the three weeks we are at home. I normally book bowling first thing in the mornings on a special offer for a penny for some days. The cinemas usually have kids club where adults go free for older films, and lots of trips to the park/ paddling pools, bug hunts in the woods, etc.