Tour Guide Recommendations for Colosseum

I'm not looking on their site to see if they offer a tour including the underground areas specifically, but we have always had very good luck with LivTours in Italy.

Welcome to the forum, Cathy.
It’s been several years, but we used Walks of Italy to tour the coliseum and forum.
https://www.walksofitaly.com/rome-tours/

You can use the search feature for reviews from the forum. Here are posts for the past two years: https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=Rome+coliseum+tours

I’d avoid 3rd party vendors such as Viator. If there are problems it’s much easier to deal with the tour provider.

At a glance, neither Walks of Italy or LivTours offer a tour of the Colosseum that includes the Underground anymore. That's likely because tickets that include that piece are so very difficult to get that they, along with some other companies such as Through Eternity, have thrown in the towel on trying to do that altogether.

You might try The Roman Guy: their tour which includes the Underground also includes the Palatine and Forum but comes with the caveat that there is the possibility of it being cancelled, moved to a different day or, likely, a different time so you have to leave your entire day open in case that happens: read the FAQs carefully:
https://theromanguy.com/tours/italy/rome/colosseum-dungeons-tour-with-roman-forum-palantine-hill

Otherwise try to get tickets from the Colosseum's official website...but it's not easy as they can sell out immediately upon becoming available.
https://ticketing.colosseo.it/en/categorie/singoli-1-8-persone/

You can see down into parts of the Underground from different locations above so if you have to give it a miss, you won't be missing all that much, IMHO. You and your young traveling companion will find so many other interesting things to see in Rome! :O)