Hi,
All the above are great for kids. A couple of books that are really good are the Rome Reconstructed ones - sold everywhere - where they have pages with photos of how the ancient ruins look now and a plastic page you layover the top to show how it was - they are great to help children (and adults) imagine how the buildings once were. Also 'Ancient Rome for kids' by Parisi, (I bought ours in the bookshop at the Forum), has great cartoon illustrations and lots of info that kids find interesting.
A couple of other things to do - if they like cats a visit to the cat sanctuary in Largo Argentina. The Baths of Caracalla are also very good. There is a cannon that fires from the top of the Janiculum hill every day at 12 noon. There are great views of the city from there and a puppet show at weekends just after the cannon (bit violent though sometimes - think old fashioned Punch and Judy style). It is nice to see the canon then walk down the hill to Trastevere and have a pizza (nice part of town, narrow cobbled lanes and lots of restaurants). Also in Trastevere is the church of Santa Maria - a beautiful church with great mosaics that the children will also enjoy. The Mouth of Truth - explain the legend and see if they will put there hands in! (My son when 5 refused on the grounds that he certainly did NOT tell lies but the 'mouth' may make a mistake).
If you think they would be interested a trip to Ostia Antica, the old Roman port (similar to Pompeii), about 20 minutes on the train from central Rome. My DS (now 6) really enjoys a trip there, you really get a sense of what it was like in an ancient Roman town.
If you are going when it is actually Easter bear in mind it is very, very busy, particularly around the Vatican. If you go to the Colosseum it is a good idea to buy the tickets on line, a couple of euro extra but worth it to avoid the queues. Also, carry either one of your passports, both the children's, or some form of identification that proves you are EU citizens (I am presuming you are). Children get a reduced rate or free at museums etc but often only if they are EU citizens, and usually they won't just take your word for it. With no ID they will probably make you pay the full adult price.