Apart from sensible considerations already mentioned (sensible shoes, Rome is tough on the feet) the one thing I'd say is not to automatically expect to be able to pay by apple pay, revolut, google pay, contactless, or any other digital form of payments. This was the biggest shock to my daughter, born and brought up in Italy, now at university in England- that cash is fast disappearing. Here it's still very much King.
Obviously, most places will accept contactless, and according to Italian law, must. But you'll still find a LOT of places that will either not accept digital payments, or will impose a minimum amount to do so. So, some cash will be handy.
I take kids in groups in the other direction and the information I give them is: nothing worse than a backpack for advertising you're a gullible tourist- anywhere, not just Italy. Likewise a phone in a back pocket. Not sure what a teenager would accept as an alternative though! Only on Friday there was one of the regular news items about pickpockets around big tourist sites- the most famous are NOT African immigrants, but very normal looking teenage girls. Operating in giggling groups. I've been pickpocketed in Florence (many years ago though) but never in Rome.
You won't be allowed to pop into a McDonalds to use their loo. They've long cottoned on to tourists doing that (a clean loo is always a lovely thing to behold) and you can't get into them (also Starbucks etc) without the code on your till receipt. Speaking of, it's illegal for any retailer not to give you a receipt and it's illegal for you not to take it. Even if it's something for a euro. You'll find a multitude of retailers not giving them obviously. Prices in these places are, however, fairly standard and the same as the UK, whereas sitting down in a cafe for a coffee, you'll pay waiter service.
Rome is very cheap to eat out, and eat well, although am guessing they'll probably be half-board or something?
If they have any free time, avoid the area around Termini station.