If you are visiting the Leesburg area, a great place to visit with kids is Pump It Up. This inflatable gym has tons of moon bounces and large inflatable slides. The gym is not open to the general public 100% of the time (because they host birthday parties and such), but it does offer frequent "Pop In Playtime" sessions that anyone can attend (visit website to see specific dates and times). The cost is $10 per child, and I believe there is a sibling discount for additional children.
There is a designated area for the very young kids who might otherwise be afraid of the larger equipment. Adults can bounce, too! Everyone must sign a waiver and must wear socks, but if you forgot them, you can buy some there.
A nice, air-conditioned activity for rainy days, scorching hot days or just any day! Last time I visited, they also had ride-on toys that the kids enjoyed using.