How much do they want to do tours and how much explore by themselves?
The Coliseum without a guide is actually a bit dull - go in, it's big, wlk round in a circle, you are done. But we saw lots of people being entertained by tour guides.
The Forum is huge and really interesting with a guide book - can spend ages working out by yourself what everything is.
I think we spent a day just doing the Forum and the Coliseum, plus surrounding area - DH points out my feet were falling off - all self-guided. If going self-guided the queues are less to go into the forum than the Coliseum but advance booking is better.
There were loads of boys doing the Capuchin Crypt.
There is no need to speak any Italian in Rome at all as there is a tourist on every corner looking lost with a map. If in doubt, you just follow the other gang of tourists - worked for us doing the Catacombs on the bus. BTW 2 catacombs is enough. I did 3 and despite being crazy interested in history and early Christianity, my interest had massively faded by the third.
Should prebook the Vatican and the Galleria Borghese, Vatican is amazing and just has loads of everything. Galleria Borghese is more 'arty' but v enjoyable.
The giant head and foot of Constantine which appear on every documentary are in the Capitolline museum. They might get more out of this with a tour as it is basically all the highlights of Roman art in one museum. Every single documentary goes to this museum to look at something and most British stately homes have a copy of something in it.