I've never been to Longleat, but have been to Whinfell, and we're going back there this summer. I'd assume they are all similar though in terms of activities.
We got the activity booking form in a few days ago, for M (who'll be 2) there's messy play sessions, something called toddler time I think which is a music type thing and another couple of things she can do. There's also soft play areas, face painting every day and things like that. There's ten pin bowling too, you can get the support things to help the younger kids to bowl - you know when you put the bowl on the top and the kid can push it and it rolls down the support and down the lane - I've no idea what they are called! Plus I think there's a cinema but the times didn't suit whatever we were doing. there's plenty of playparks and stuff too. And depending when you are going, I think all the lodges have a space for a bbq so that might be an option for tea one night although i know that Luke's not ready for that, just thought more for you and your H as a wee change from sitting in the lodge, we took a wee disposable bbq and used that
You could also hire bikes and look into getting the trailer for on the back for Luke? I would also book for something more central, i had a walk around Whinfell last time we were there and it took me a few hours, our lodge wasn't far from the centre and it was really handy I thought it was worth the extra £40 or however much it is.
Hope you manage to get booked up, I think it'll be great fun and Luke will love it! x