Explora Childrens Museum
Just a 10 min walk from Piazza del Popolo this is a great place to spend a few hours, the under 12's will love this child sized playtown.
Explora Website...

Villa Borghese
Just a 5 minute walk from Piazza del Popolo, Villa Borghese is a free wifi hotspot and offers, a zoo, bike rental, boat rental, cinema, theatre, museums and restaurants. 
Villa Borghese Website ...

Time Elevator
The Time Elevator is just off via del Corso near the Piazza Venezia end and is definitely worth a visit for both children and adults.
Time Elevator Website ...

Ice Skating at the Rome Auditorium
Just a 10 min taxi ride from the Spanish Steps you'll find the ice skating rink at the Rome Auditorium, open 08/12/2007 - 03/02/2008
Rome Auditorium Website ...