There are good playgrounds (we have been in Sherwood twice and Longleat once, and one in Netherlands also) and a sandy "beach" on the edge of the lake (not great for bucket and spade stuff, but ok, and there are fish in some of the lakes).
Usually there are various types of boat (canoes, pedallos, sailboats) available on the lake (for an extra fee) which are not clear on the website in advance. We took DD out in a 3-person canoe between us when she was that small - nowadays, she wanders down to the boathouse and rents a sailboat to enjoy herself (she's a competent sailor!).
Sherwood had a good nature centre, Longleat was ok. I remember a trail behind the Rangers' Centre which had posts along the way with various animal prints that DCs could see and take rubbings of. And lots and lots of birds and squirrels!! The rangers are always interesting, there are paid trail walks but also there are usually a couple in the building willing to just chat about nature and there are books to read etc - so a good way to pass some time a few different days. (In Sherwood, the ranger centre is near the golf course and Country Club restaurant - slightly more "proper dinners" style food (decent pub grub and carvery roast style), but also had a good kids play area, and was less busy as away from the main centre.
We tend to have bread delivered in the mornings for breakfast, some days it's just bread (different types on different days), others it included pastries etc. We would have 1 meal "out" most days, but eat "at home" the other meals - it varied between lunch and dinner out, with traditionally 1 breakfast in the Pancake House, and occasional coffee/cake or icecream stops as well. Eating at home was easy meals - BBQ in fine weather, but otherwise nice but simple to prepare food. We'd bring some food with us (coming from Ireland, we tended to stop in a Sainsburys/Morrisons en route for nice food and some basics, and some gin, wine, beer) and also go to the onsite Market some days - plenty of choice if a little on the expensive side.
We had things like lots of fruit that we would eat, crisps and biscuits for snacks, lots of cheese (we are a cheese-loving family) and crackers, olives, carrots (for making sticks) and hummus, a fajita kit, a bag of pasta and jar of sauce, salad things, butter, teabags, ground coffee, nice Seville (bitter) marmalade for breakfasts, squash, some cans of fizzy drinks, small bottle of olive oil etc.
I also brought a "self catering pack" from home, as we go SC a fair amount - small bottle of washing up liquid and a handful of dishwasher tablets, tinfoil and cling film (and usually remember a few sandwich bags for marinating things and bringing snacks to the pool), salt and pepper. (I usually bring a few pegs as well - clean ones to be able to use closing food bags, but also to clip things to the chairs outside for drying). And a small decent sharp knife, corkscrew, veg peeler, tin opener and 1 small wooden spoon. (It's surprising how many corkscrews we have acquired over the years from having to buy them for SC houses, and we have multiples of most of those utensils for the same reason). I also bring 2 new J cloths (happy to leave behind at the end) and a handtowel and teatowel for the kitchen (that come home with me) so I can alternate with the CP ones when they get wet.
We often go swimming about 4.30 or later, as the pool is starting to empty out - have a decent snack in the afternoon, and have things organised to do fast dinner/booked table afterwards - DD would be hungry but then would happily go to bed after eating and we could eat in peace and relax (if we ate at home). You are allowed to bring drinks and snacks into the pool building, just no glass. Flip flops or crocs are very useful! And a small backpack for swimming gear/towels, Parc Market purchases, bringing snacks/drinks on walks and bringing back cones/sticks/drawings/… from those adventures! As there is a lot of walking (cars must go to the car park outside the park but inside the perimeter after you unpack on arrival until you are packing up again for departure). Bringing or hiring bikes CAN be a good idea but it can be as nice to just walk.
Most houses have a bath - so bring some nice products to enjoy some relaxation time. And let DCs enjoy splashing as well (if they have a favourite bath toy, might be useful to bring along). And also any favourite DVDs for playing in the house for quiet time. There is usually some chalk for the big blackboard, but we bring more as we drew lots of pictures and also used to leave notes to each other (shopping lists, where someone had gone if we went different directions so plans to meet later or reminders of activities we'd booked...the odd love note...
). We also always bring a deck of cards as we tend to play lots of games as a family when we are there - DD does some activities, and occasionally so do we, but mostly we enjoy a relatively simple existence for the week, lots of walking and swimming and fresh air, good food, a few drinks, reading and games....
Bring waterproofs - some of our best fun was wandering through the forest in the rain, and DD jumping on molehills that popped up! And seeing the various animals around no matter what the weather.
There is a launderette generally - you don't necessarily want to need to use it, but handy to know (around guest services near the entrance normally). I usually throw in a couple of washing powder tablets from home into my "pack" as well - just in case.