I don't have kids myself but have done a lot of AI holidays - I used to like alternating between full on, do it all yourself, walking all day city breaks and then have the next holiday a week AI just to relax. Some have been with family/friends children and I can also remember holidays when I was a child really well so some of the tips should still hold!
Kids club, if they go at the start then they will likely love it. I used to love kids club and I'd make friends who I'd spend all of my time around the pool with.
As a previous poster said, don't stress about junk food and fizzy drinks etc. Your children will very likely start to tone things down and self regulate after the first couple of days. Knowing you can get an ice cream or Fanta whenever you like is a huge novelty!
If budget allows and you're close to the town, I like to have 1 or 2 nights of the week to go out to eat somewhere local, look around the shops etc. Look on trip advisor for good restaurants near to your hotel/near to the centre. It's a good way to break up any monotony, a chance to look around the shops and also a chance to eat some lovely food.
On the days when you're just in the hotel- don't be regimented, just go with the flow. Grab a snack, have a shandy, relax. Don't stress about missing lunch, there will always be snacks available. If you want an early dinner one night, you can!
As much as I love city breaks and planning them to within an inch of their life (!) there is something to be said for a week AI where you don't have to be somewhere at X time and you can just relax.