Hi MJ,
My family and friends (6 of us in all) took the Eating Italy food tour of Trastevere this past June. It is an amazing tour, our only regret is that we did not take the tour earlier in the trip as our two girls wanted to go back to a few of the places that we closed on Sunday and then we had an early flight on Monday.
Going on a tour early in the trip has the advantage of being able to learn about the local food scene and the guides are knowledgeable about all parts of Rome and eating well on a budget. The Saturday evening is going to be tough for you unless you take a nap in the afternoon and then it is probably going to be fine as this tour is on the move and very engaging. The only advice is go hungry, because it is really a lot of food. Eating Italy also allows up to 48 hour cancellation and you can get cancellation insurance (for a very modest fee) that allows you to cancel at the last minute if you decide not to go.
Note: on the nap front, lots of debate here on the question of napping. When I travel alone or just with my husband, we try to plow through to local dinner time. When I traveled this past June with my two friends and our two college-age girls, we arrived at 9:00 a.m. in Venice. I had arranged for us to check-in early to our flat. This was the best decision of the trip. Dragging 6 people who are not only jet lagged, but sleep deprived through a foreign country would have been a nightmare. We checked in, got showers, and then went out for my friends birthday dinner and everyone was in a much better mood. I have traveled with 12 and 15 year olds (to France) a number of years ago and tried to power through, the two boys did not do well with that and we had a rough start to our trip.
You know your family the best, but if you can arrange for an early check-in, then if you need a nap, you can take it.
Have a great trip, you will love Rome and Venice.