Parc Market has child-sized trollies, which most small DCs love to be in charge of! So do plan on going for a couple of things at some stage.
There's not a lot of clothes drying space for wet towels etc - but there is a launderette near the entrance reception/office block which is handy. But with smallies, bring plenty of towels for swimming and if you have more than 1 set of togs, bring more than 1 - there are heated towel rails in bathrooms in the chalets, but having a spare dry pair can be handy. Bring a bag for swim gear too - there are lockers to put them in but it might be a bit of a walk from the chalet to the pool. There are plenty of buoyancy aids for non-swimming small DCs, and the kids pool area there is nice - a great slide from a pirate boat, fountains and nicely warm too for babies.
We tended to bring free printable colouring sheets and crayons, pack of cards and a board game when DD was that small - now it's DVDs and her ipad. There is a chalk board and chalk in the kitchen, which is fun for drawing on (maybe bring some spare chalk if you have it at home).
The nature centre is nice for a quiet walk away from the bustle of the pool area, and there are various talks and wardens to give advice and chat there a lot of the time. There are poles with various animal shapes to do rubbings of on the walk which your older DC might like to do.
There's also a good few play areas dotted around. There's a big one near the beach of the lake, but there are others scattered around the site. If I remember rightly, there is a small indoor one in the Foresters restaurant near the nature centre, and probably a few other restaurants - but that is nice as it looks out on the golf course and is away from the main centre.
They will give you a few dishwasher tablets - but if you have one at home, bring a handful to be able to run it as often as you want.
I'd tend to bring one easy meal from home that you know the DCs will eat (the makings of 1 dinner) and a few snacks (tinned spaghetti, beans, tuna, etc) for lunches. And some nice nibbles for yourselves once DCs go to bed (nice chocolate, cheese and crackers, decent crisps etc) and maybe something nice to drink (really good hot choc, nice bottle of wine, or a few beers). There are plenty of choices in the parc market but it is a bit limited and pricey so if you have particular favourites, it's handy to have them with you. Also, we like to bring proper ground coffee with us for nice breakfasts, and get fresh croissants at the market.
Bring sunscreen and rain gear. DD absolutely loved stomping on molehills in the pouring rain when she was young!! Lots of people don't venture out as much in the rain but it is lovely once you are dressed for it.
Enjoy!