Don't fall into the trap of booking loads of stuff and then finding you are so busy you don't get enough time in the pool!
I would say pick one, possibly two activities a day and leave yourself plenty of time for the swimming pool.
Kids love the panto! Parents less so. I would say book it, it tends to be one of the cheaper activities too and the kids are mad for it.
We did mammal magic last time we went and that was great. I think your dc's ages are about right for that if they like cute fluffy animals which they can get up close too and pet. It's new though so not sure if they have it at all the villages (we were at Elvedon)
Definitely take or hire bikes and go on bike cycles round the village and not just for getting from A to B. You might want to consider a tag along bike for your youngest and if you do be sure to book it in advance as they go fast.
Have you booked a specific lodge/area and is it near water? If so you'll have ducks and geese to feed and the dc will do that for hours! Stock up on plenty of value loaves to take with you (the stuff in their shop is far too expensive to give the ducks!). I always take 2 loaves per day per child and still have to ration it to 1 in the morning 1 in the afternoon! If you're not near water/ducks then still take your bread up to the dome where there will be ducks and moorhens to feed.
The treasure hunt is good but rather long and gets a bit tedious for smaller ones. You might be better doing the easter egg hunt which I saw on the website. Or even would say you may be better off just going for a bike cycle.
The parks are free and will keep little ones busy for a while if the weather is nice. We also do a trip out to the pancake house and for ice cream. It's not cheap so that counts as an activity for us!
Also perhaps think about having a go at crazy golf or hiring a pedalo/row boat or even ten pin bowling. Avoid the games machine room though unless you want to spend a small fortune!
Can't advise re the clubhouse as I tend to feel that we're away as a family so we do things together so don't want to stick ds in the time out club. Much as I sometimes think I would love to go for a massage or spa session.
My ds is a bit older but we went with a friend one time and his 5 year old enjoyed the crazy coasters as much as my (then) 10 year old ds.
We also did the spy challange last time (ds now 12) and that was really good and more manageable than the treasure hunt in terms of cycling round the village looking for clues.
We enjoyed it so much I've already booked that again when we go in April.
Final bit of advice... Don't book too much, not least because you'll find the dc expecting loads of extra activities next time you go! But if you do want to do actiivities book them soon as they will fill up and you're unlikely to get anything much if you leave it till you get there.