We had a couple young Adventurers on our Viva Italia trip and they did quite well, but there were several days that they were wore out and went back to the hotel with parents, so they all wound up missing part of the experience.
You have to figure the combination of heat, crowds, and a LOT of walking/standing will run even the best travelers out of gas. Also, although
ABD does a great job of making things kid-friendly, there are still museums, cathedrals, etc. that you just don't appreciate until you're older.
You also have to consider with group travel, you don't want to be the ones holding a group back or monopolizing the attention of the guides. Even though a "mini-meltdown" wouldn't seem so bad to me, it might not be endearing to all my fellow travelers. The youngest in the group also are probably going to feel a little "left out" by some of the older kids.
We purposefully waiting until our youngest was a few years older than the minimum age so that:
- She'd remember everything,
- appreciate the cultural aspects,
- be able to keep up with group,
- have the palate to experience the variety of food.
You know your kids best, but you want them to enjoy it as much as you. The Colliseum has been there for thousands of years and it will still be there in couple of years!