back to the Butlins idea if it helps -> 5 and 3 are the perfect age really. Any older and you have to start paying for activities, any younger is a bit too young (although as ds2 is 7 months he didnt really care - he liked the baby soft play bit though)
Favourite things for DS (3.4) were the shows - pretty much all of them were a hit, but we took him to every bob the builder show (20 minutes on every morning) and he also loved the skyline gang (butlins own characters) also Thomas tank show, pingu, angelina ballerina, billy bear. There were a lot of characters he missed due to being asleep or wotnot - like loony tunes and diego.
They do photoshoots with characters too but you can take your own photo rather than pay for one. I wouldnt fork out for character lunches/breakfasts either because you seem them about all the time and they are brilliant with littlies and always stop for a cuddle and a photo.
In the nursery they have messy play sessions each morning which to be honest is just a chance for them to play somewhere else other than pavillion - DS loved it and raided the dressing up box - ignored the paint and water!
They have a stage in the pavillion which is open, then centre stage which is ticketed (tickets free, but you have to collect beforehand) Also had a character olympics outside during the week where they could take part in egg and spoon races etc with Fireman sam and others. The mini funfair is good for the littlies - planes, teacups, ladybirds and noddy car rides.
The soft play area is massive - there are two bits - under 1m and over (DS got into both cos he's pretty much exactly 1m) Also outside the sun and moon pub is a large wooden play area. If they are tall enough they can do some of the things in the big funfair eg trampoline and carousel. DS was too small for dodgems though. The pool was a hit because of the slide area for toddlers, although its not enormous so could get crowded during school hols.
What else. Um. You can walk into Bognor if you really want to, or pay for the mini train that leaves from outside(not necessary to take it as its only a few mins walk, but DS enjoyed one ride)
There are plenty of places to eat on site if you want to, some of which are reasonable, some are not - or like I said head to the local tesco and stock up.
There are arcades and lots of those grabber machines, as well as toddler rides which cost a pound. DS liked to get on them, but we never needed to put money in cos he just likes to pretend!
Hope that helps