This is why I went LAST week!
Unfortunately due to the fact that the school holidays are now staggered, it is very difficult to gauge which week is going to be busiest our of the two. I booked my accommodation at the start of this year and I did cheek a few counties for when the majority of kids are most likely to be off. Unless I go thought every single county and total up the exact numbers of kids off, plus the information displayed on the web at the start of the year was accurate at the time of viewing and could change, I myself would have chosen to go last week.
Despite our local schools were off last week, Norfolk attractions were still very quiet.
As it stands, I'll be on park tomorrow and Wednesday and I'm not looking forward to it. However from experience, I should still be able to get on all the rides that I came to do, but will have to plan my day carefully.
Next year, I will have to check out every single county in the UK to get an idea when the majority of kids are off school.