I don't have any specific advice on tours as my visit to Rome was several years ago and non-cruise, pre-kids. You might also look into if there are some private or semi-private non
DCL Colosseum tours available that are designed to be extra fun for kids.
That being said, I just wanted to say that I think your itinerary sounds great and I think it's wonderful that you are exposing your kids to historical/cultural sites despite their young ages. My kids are 4, 5, and 7 and I can say that based on our travels, each of my kids WOULD get a lot out of a Colosseum/Rome visit so it's wonderful you're not "selling your kids short" and skipping great historical sites just bc they aren't stereotypical kid activities.
I think most kids can appreciate historical sightseeing if they are exposed to it young and the parents/tour guide explains it in a way that makes it come alive. You might also consider finding children's books, audio books, paperdoll activity books, etc. about Ancient Rome before your visit so your kids can start getting excited and Rome will mean more to them. Playmobil also has some Ancient Rome toys (chariots, Colosseum, gladiators, Roman family, etc.) that my kids love and might make fun gifts for your family.
Enjoy your trip!