Olim, My children are 2 and 5 years old. Most of the children there were between 4 and 8 years old, but there were some older children too (up to about 12/13 years old). I think it largely depends on the luck of the draw when you go and who is there. You may luck out and your 8 year old find a perfect holiday buddy, but equally you might not be so lucky.
I had booked for 1 week and was considering extending it to two, but I decided against it. Firstly because it was my first holiday as a single parent and whilst it was lovely, with lots of activities for the girls, and some time out for me (probably more so than any other holiday), I came to the conclusion that a week was perfect for us.
You're not tied to the site at all, but the food is so delicious that you tend to stick around for that! I used to live in Italy and still have family, so I did drive to various places - Cortona, Arezzo, Lake Trasimeno. I didn't venture into Florence or any of the larger cities as I figured the girls were just too little to appreciate it.
I would say about Villa Pia that you would either love it or hate it. We LOVED it. But if you prefer a less sociable holiday, it's probably not for you. In my opinion, the website really doesn't do justice to the place.