Rick Steves app has a fun kid friendly guide to the Colosseum - I read that out to my DC rather than use a booked guide, as it was more flexible.
Book Colosseum as far in advance as you can (2 months I think) through official Coop Culture website. Try to get tickets with Arena included as that’s the bit where the gladiators actually stood, plus you can see down into the vaults below.
If it’s boiling, don’t bother with the Roman Forum/Capitol Hill. There is no shade. We visited in 34 degree heat and kids were not impressed.
Pantheon is a must. Trevi fountain too - although it’s packed with Instagrammers.
There’s a fun old fashioned toy shop at the north end of Piazza Navona. Campo dei Fiori is quite touristy but there are some stallholders who put on a bit of a “show” selling their stuff - our DC really enjoyed.
Gelato - don’t worry about going to places with top reviews/long queues. It’s all good!
If it’s not too hot you can walk or bike ride down the Appian Way south of the Colosseum.
Again, if it’s hot, make sure wherever you choose to eat has air con, before you settle in! Or if eating outside, that your table is in the shade. Also prepare to use taxis more than you might have planned, better than a long walk in the sun.