We actually went inside the house at Knole for the first time last week. I don't recommend it for a spirited 3 year old as it is very much a 'don't touch' type of place. Thankfully they have a deer hidden in each room so it does give you a chance to have a look while your child is looking for the deer, so long as you can get them to stand fairly still while they look. We were mainly going in because Kit wanted to have a look, we weren't inside for long. He did manage to get dressed up as a Tudor though, which was fun. And he did enjoy looking for the deer, so as long as you manage your expectations it probably is worth a look around the house if you go and are a member so it doesn't cost you.