Yes, we purchased our tickets already for the Coliseum. We used the link that RIL provided in their email. It led us to a combo ticket that included the Coliseum/Forum/Palantine Hill. You will save a lot of time if you get them in advance
Cindy posted on facebook that these sites should be free for kids under 18. I keep meaning to check with the first
DCL Med cruise to see if this is in effect yet but I haven't located their Dis page yet to ask them

We already bought our tickets but for those of you that haven't it would be good to know if you need them for the under 18 cruisers.