I've been a couple of times, once at christmas time (not through choice I might add) and once off-peak season, which frankly was vastly better.
I took my 3 year old along, must admit she loved it and was knackered every day thanks to the pool - meant a nice early night and plenty of time for us to relax! I'd recommend it, just remember that you're only 20mins drive from towns with supermarkets, so feel free to pop out and get stuff, but you can't bring the car back to the accommodation so you'll have to carry anything you buy.
Top tips:
1. Book early, try to get somewhere near the center or one of the pubs. Sometimes costs a little more but it is worth it at Cumbria.
2. Arrive early, ideally you want to get to the house, dump everything and get the car back to the car park before the rush. Just chuck wife/kids into the house and get it done or it'll take hours. Military operation style, this is where a house near the car park or central area helps a bit.
3. Buy the bricks for the fire from aldi, far cheaper than on site
Spa day - if you can get away with wife is worth it, expensive but trust me as a bloke who's never been to this sort of thing the whole world of spa was fantastic, never felt so chilled out! Was an anniversary treat for the wife
I've only ever been for 4 days max (Monday - Friday morning) I'd have managed a week no problem though if the weather was holding up.
Horses - if your wee one wants to go on the horses as mine did it's a LONG walk to the place to leave early and it's all up hill. There is a toilet at the top but take something to drink.