Villa d’Este in Tivoli (Italy) is a great place to visit with your kids.
A lot of interested fountains, many beautiful views, shade of big trees are perfect for trip with kids in hot summer day.
Go to metro station Ponte Mammolo (blue line B), take bus to Tivoli (1/2/3 platform from the left side). You can use your daily ticket, it works on this bus as well.
Expecting arriving time without traffic jam is 40 minutes.