One thing to keep in mind, you'll only be able to do one major sight per day. So, for example, if you want to see the Colosseum, Sistine Chapel, St. Peter's Basilica, and the Roman Forum then you will need at least 4 days in Rome. That general rule of thumb can make your planning easier.
Also, to get your kids more excited to see some of these places, try to find some movies to show them before you go. Gladiator is very violent and not meant to be 100% historically accurate. That movie would be a parenting call as to whether it's age appropriate. But the Coliseum is in there and it's definitely entertaining. I haven't seen anything from the HBO series Rome, but I think it is also quite violent. Although there are those who would say that Rome was marred in its decline by severe violence.
I'm sure there are some other movies out there that aren't so violent. I'm not a big movie buff, so I'm not very helpful there.