If it is dry (or dry ish) and children will all either walk or be carried (in backpacks/slings) then there is a walk around part of the Longleat Estate which includes seeing the view from Heavens Gate and the Sculpture. Photos
Absolutely not, one day can do everything with ample time left over. I have travelled to Longleat from London several times and done the whole safari park, attractions and house in just a few hours. You could always get a day pass to Centre Parcs to then do some other activities?
Visit Europe, 12 countries in 3 days! Of course you could do it all in a day but why bother? You can see most of the pictures on the internet anyway. I you want to just visit, cram it all into a day. If you want to enjoy the experience, get a passport and split your time, visit again later in the year.
You can spend an hour waiting to get on the boats so factor this in too. My tip is plan your day(s) carefully. Get to the boats first thing before the queues build up. Book a trip on the safari bus so your car doesn't get wrecked (although the monkey enclosure is closed at the mo) and your driver will enjoy too. Take your time and don't rush.
I get a season pass every year, and usually make three or four days out of it! Things like the adventure playground can be used all day, you get the children's hand stamped. If the weather is hot, there are water jets that shoot up that the children can run around in and my kids can literally spend hours there. Also, you can buy a pass for the second day for £3 per child (but children under 3 are free!) Also, if you get the bus to go round the safari (which I prefer) it takes about 2 hours to go round although you have to pay more for the safari bus. The safari is best done later on in the afternoon - less queues and you tend to get a better view of the animals then as they start coming out for food about 4pm.
I'd also avoid going at the height of summer when the queues are just ridiculous. We tend to go in the months leading up to may and then finish off our pass in Sept, but I live reasonably locally (takes me an hour to get there), so can do that.
Also if you have Tesco vouchers then I'd get them through that - it's much cheaper.
I certainly couldn't / wouldn't do it in a day - the hedge maze takes about an hour to do, the safari at least an hour - that leaves another 10 activities to do - so if you want to fit them into one day, you'd have to do two activities an hour! But when you consider they can easily spend three hours in the soft play / adventure castle bit, another hour and a half in the animal pet centre and so on... its easy to fill two days and still not get everything done! HTH