I go to Longleat quite often. Have not been at Xmas time for a few years now, so am going just before Xmas this year.
At Xmas time, wreaths are put on the villa doors. There is an Advent Calendar thing in the plaza - if you get up early enough, you can get the chance to win that day's prize. Christmas decorations, lights for example around the outside pool area. In village square there was a Santa's Grotto, which I think had singing reindeer.
I can't work out the difference between new style exec and old style. If it is of help: Video: New Style Exec 1 Bed - Video: Old Style Exec 1 Bed Apartment - Video: 3 Bed Woodland Villa
I tend to avoid the food in the restaurants generally. The Indian is very nice. Cafe Rouge service is so slow that I've not managed to ever eat there... I tend to leave after 5 mins of not having anyone even give me a menu.
Sports Bar I find is quite good for what they offer. It isn't great food or great service, but you do get hot food in a reasonable time.
The Center Parcs website has menus for each restaurant showing prices.
I tend to self-cater and buy some bits from Morrisons (2 miles from CP) on the way there and then get other things from the Parc Marc.
Parc Marc prices (from March 2009)
Stir Fry mix - 2 for £5
Semi Skimmed Milk £1.59 (think that is 4 pints)
3 large onions £1.59
Breakfast Juice 1litre £1.49
Small bag of sugar £0.70
Anchor Butter £1.29
Chicken oxo cubes £0.69
Pullet of Seedless Grapes £2.75
6 Large Free Range Eggs £1.45
Hope that gives some idea of costs at Parc Marc.
Center Parcs seems the same level of busyness whatever time of year I go. Have been August, October, January, March, June over the past year.
Activity prices vary. At 4.5yrs I would say that a lot of the young children activities are probably best avoided. He may far prefer splashing around in a stream, making dams, pretend fishing in the pond behind your villa. Swimming of course will be something you will do during the day and early evening - nothing like being in an outdoor pool when it's dark.
Activities he may like I feel are: Mini Golf (£3.50 child, £4.50 adult 9 Hole), Indoor Climbing Wall (£5 for 15 mins - Min height: 1.1m), Rollerskating (£3.50 child, £4.50 adult - 45 mins), Ten Pin Bowling - about £20 for 45 minutes, up to 4 players.
Prices are subject to change. Prices taken from Longleat Village Guide published 03-Apr-09 (valid to 04-Sept-09).
Many activities can be booked online a few weeks prior to your visit and whilst on your stay.
Which accommodation?
On my own I book Executive. With others recently we've had Woodland. Have had Comfort in the past as well. All are fine. Exec come with WiFi, so I can mumsnet whilst on holiday!
Which location?
If not hiring a bike, then aim for Pine/Ash/Maple. Fir is the furthest away from the Plaza/Swimming so it is a bit of a trek, though there is a Land Train service on Sat/Sun.