Rick has some walking tours in his guidebooks; there's a book for Rome and a book for Florence/Tuscany as well as the full-country Italy book. Since your stay is short, you probably don't need more than "Italy", which would be less expensive than buying the other two books. Do be aware that opening days and hours of sights are still in flux, so you'll need to go to the websites Rick has in his books to get the latest information on places you want to see. Some sights change their hours as the months go on, winter to spring to summer, so do pay attention to the details.
To get Rick's audio walking tours, download Rick Steves Audio Europe from the Google Play Store or Apple's equivalent. Then add the tours you're interested in. He has five tours for Florence and nine for Rome. At least some of those tours would probably be worth listening to for historical background before you even get on the plane.
I do not equate a recorded walking tour with having a live guide, but I think you're right to be cautious about overloading on tours, especially when there are children in the mix. Some tours can be quite expensive, too. I do think a recent suggestion that the Roman Forum is a very good place to have a tour is smart. When you're looking at ruins like that, it can be difficult to understand what you're seeing without someone to explain. And of course you can ask questions of a guide.
I don't believe it's generally difficult to buy online tickets in advance yourself, and a timed online ticket is automatically a "skip-the-line" ticket. Everyone has to go through whatever security procedures the sight has implemented. I confess that I haven't yet purchased my first online sightseeing ticket for Italy, though, so there may be wrinkles I'm not aware of. I do have a general tip: Failed online purchases can be due to your credit card's security procedures. If you have a problem, call the number on the back of your card and tell them what purchase(s) you're trying to make in Italy; they may ask the expected amounts of the purchases. If that still doesn't solve the problem, try a different credit card. Sometimes switching devices (PC to tablet to smartphone) helps, but I believe most issues are related to credit cards.
Sightseeing tickets tend not to be changeable or refundable, so timing of ticket purchases is sort of a balancing act. You don't want to miss out on seeing a place on your target list (or to have to buy an expensive tour to get in), but you don't want to commit a bunch of money up front and then perhaps have to delay or cancel the trip.
If you want to see the Vatican Museums (which include the Sistine Chapel), you should check ticket availability for your Rome dates now if you're traveling fairly soon. That might be something you need to jump on right away. This is a place that can be mind-blowingly crowded (though probably not as bad as usual just at the moment). The usual advice is to take a tour (costs extra) that gets you into the museums before they open to the general public or to go late in the afternoon on Friday or Saturday, when they are open late (that doesn't cost extra). When I looked at tickets for late April yesterday, there was little available before late afternoon (not desirable on the days when the museums close at 6 PM). In other words, it was looking really tight.
Edited to add: Here's the ticket website for the Vatican Museums: https://tickets.museivaticani.va/home/calendar/visit/Biglietti-Musei
If you just want to see St. Peter's Basilica, that is free.
When is your trip? Is it during Holy Week? There will be a lot of extra people traveling then, looking for tickets to things. Demand may also be higher the week after Easter. And then it's practically May, when demand always increases.
