We first went when Ds1 was 18 months old and we go every year. We are booked again this summer and he is now 22 years old. 
Every check in day we go to Hucks as it is our tradition. It usually opens at 4pm but opens at 12 on check in/check out days. There are always play areas in the restaurants. Make a note of all the opening times for Parc Market etc in case you need anything or want a Starbucks or whatever. Definitely book into a restaurant, they will be busy.
We always arrive around 10.15am, collect bikes at 11am which is the earliest you can collect them (I now hire an electric bike, complete game changer so I can tackle any hill) and when the children were little we did hire the yellow trailer to be able to transport them distances plus they loved being in the back of them.
We cycle to the swimming pool after bike collection as we always have a backpack ready to go for each of us. We have a swim, the toddler section is fantastic, not now of course but a great place to play for little ones. We also do early access to the villa for 2pm which you can pay for. Ds2 (now 18 years old) and I head to the villa, Ds1 and Dh cycle up to the car park to collect the car. Ds2 and I unpack (packing cubes so easy) whilst Dh and Ds1 return the car.
There is a lot you can do for free, wildlife is literally on your doorstep. We have a spreadsheet to remember what to bring. I agree on taking an air dryer although I think they provide one, we take 2 sets of towels and swim wear. There are lots of videos on youtube of people at Whinfell. I hope you have a wonderful time.