How to find, research and book a private tour guide for a day - family of 5

Have you looked at RS latest books on the areas? He listed private guide resources in them. We did one for Siena and Orvieto a few years ago and were very pleased with them.

We’ve had luck finding private guides at toursbylocals.com. Their website has a cruise excursion link. You plug in your cruise line and dates and each port pops up with corresponding guides. The prices are also listed.
We just used a guide through their website for a day trip to Sintra from Lisbon last Thursday. It was a lovely day and our guide was wonderful.

Way back in 2015, I did a one-day tour with Laura Berti of Florence for You. She put together a day for us that combined Medici sites + Mercato Centrale + the Oltrarno + artisan workshops. She is a native of Florence, so she knows the city well. From looking at her website, it looks like she is still going strong, and she has a Florence for Families tour (though I'm not sure the age of the 5 people on your tour).

Our day with Laura was great! For the record, "us" = male and female travel companions in our early 40's at the time.

https://www.florenceforyou.com

I also have had good success with the Tours by Locals site, BUT TBL takes about a 30% "finder's fee"/commission from the guide. If you can find the guide's personal website (and often there is enough info in their profile and photos that you can), then you can often find a much better price for the tour(s) of interest to you.

You might check with Anna Piperato. She's based in Siena, but does tours in other parts of Tuscany. She has her own tour business, but also guides for Rick Steves as well as for Sarah Murdoch. We had for for a day in Siena a few years ago. She's American and British, but has lived in Siena for years. Check out her website.

https://annapiperato.squarespace.com/

We don’t take cruises but good friends take them often. They book their tours with “ Tours by Locals” and have had wonderful guides.