We were at Longleat at the end of September - will go back, but would recommend the following:
Definitely pre-book beauty treatments, and if you want to use the creche or go on the Teddy Bears' picnic I'd recommend you book those (my DS who is 2 loved the picnic, and then DH and I played mini gold and had a nice relaxing afternoon while DS was in the creche)
Eating out is expensive - every meal seemed to cost at least 25 quid, which when you're having lunch and dinner out on a few days can add up, especially when the food is, at best, average!
The Parcmarket has very strange opening hours (at least I thought so) so I'd bring as much stuff as you think you might need. All the chalets have microwaves, fridges and loads of saucepans etc so you can self-cater if you want to.
One thing about Longleat - it is VERY hilly, so if you're staying at the far side of Jardin des Sports check out the cycle paths properly - there is one with a minimum gradient, but we only discovered that on the last day!!
It's all very child friendly, and there's a Dolphins session on some morning where only under 3s are allowed in the pool for an hour, and they're given lots of toys etc etc.
Obviously bring towels for both use in the chalet and for swimming - and make sure you bring a backpack to lug stuff to and from the pool.
What else, oh yes, it was FREEZING!!!! It felt about 5 degrees colder than London was when we left, and I definitely didn't have enough warm clothes, which you need when you're out and about (especially your DS if he's going to be spending time sitting on the back on one of your bikes).
You can arrive at the Park from 10am, but can't use the pool till 2. I felt like we wasted our first day - we got there at about 12.30 and queued for about 30 mins to get in, and then by the time we parked, and had lunch and went to the villa it was about 4pm, and then dropping off the car hiring bikes etc, meant we only had time for a v.v. quick swim. I'd get there early, hire your bikes (you can park them near the pool), then explore and have lunch in readiness for a 2pm swim, and then head to your villa at 4/5 ish, and pick up your bikes when you drop back the car.
You need £1 coins for the lockers at the pool, and none of the hairdryers seemed to work, so you may need hats to keep you warm on the way home!!!!
You'll have a brilliant time, we did lots of walking and cycling (which DH NEVER does, and even he enjoyed is) and DS loved it too.
HTH
Enjoy!