I would book a guided tour on the Colosseum's website or with a qualified tour company versus trust ANYONE just hanging around outside anymore: there's no guarantee that they aren't using illegally resold tickets:
https://colosseo.it/en/opening-times-and-tickets/
They're allowing only accredited guides:
https://colosseo.it/en/2023/12/pass-guide-extension-of-validity-and-opening-of-2024-accreditation/
All tickets are now timed entry, and all are now nominative, meaning you have to provide personal information upon purchase, and your identity is checked upon entry: bring your passport. This includes your children's passports as well if entitled to free tickets (under age 18).
https://colosseo.it/en/opening-times-and-tickets/
The Underground may only be accessed with an escorted tour:
https://colosseo.it/en/tickets/regulation-for-guided-tours-in-the-underground-levels-of-the-colosseum/
The official website does not currently offer guided tours of the Colosseum, Forum AND Palatine; just the Colosseum. For a professional tour that includes all three, check out The Roman Guide, Through Eternity, Walks of Italy and some that other posters may recommend. This doesn't mean they can land tickets that include the Underground: those sell out VERY quickly when they become available. As the park is just about to change ticket providers, I'd keep an eye on the website for ticketing details, tour options and procedural updates.
https://colosseo.it/en/2023/11/parco-archeologico-del-colosseo-signed-the-contract-with-the-new-ticketing-system-provider/
The current provider does have a ticket option with an audio/video guide but it only covers the Colosseum, and you're on your own for the Palatine and Forum. I would assume the new provider will include this option as well.
https://www.coopculture.it/en/products/audiovideoguide-of-the-colosseum-with-ticket-colosseum-roman-forum-palatine_h24/