I have a great hotel recommendation in Venice: the Hotel Ala. It isn't as posh as the Marriott branding, but it's clean and comfortable and ohmigoodness the breakfast is amazing. We would go down in the morning and load up on good food and great coffee, gently stash some rolls in a ziploc for the 8-year-old picky eater, and manage most of the day before we needed to break for food in the evening. The hotel is walking distance from St Mark's Square, and the concierge there is delightful and will help you book things like boat trips. (There's a free boat to a glass factory that has a long leisurely trip out with a lot of sightseeing followed by an island glass factory/shop. Then when you leave, they boat you back at high speed with no sightseeing. Maybe the trip back would have been different if we'd bought something.) There are some great seafood restaurants there. Avorino Gelato (they have a branch in NYC) started in Venice. I still dream of that place (we were there in March a few years ago).
But your question is Rome. I traveled in Rome with my husband's 70+ year old parents and an 8-year-old. I know every McDonalds in Rome because they have free bathrooms (this is critical when traveling with that age group) and both age groups wanted McDonalds food after Mom saw the sign on the sandwich place that was offering "raw" ham sandwiches. May your group be more adventurous in food choices than mine! It made me weep to only get McDonalds and Pizza Margharita in Rome because everyone was too timid to try the food. If my husband had been able to join us, this may have gone better than it did.
The Marriott is expensive, but there is a decent Hilton DoubleTree that is cheaper and very good ratings and closer to everything than the Marriott is. But really, you're in Rome! Try some local color and get away from the chain hotels. When we were there, we stayed in a small hotel that was very close to the Basilica Di San Clemente, which I highly recommend visiting because it still has three layers of churches with the oldest layer being a temple to Mithras. Please don't miss that one. It's a gem and worth the effort. We only found it because I found a hotel within short walking distance of the Coliseum and it was literally almost across the street. I was worried about Dad's knees and my 8-year-old walking, but we made out like troopers. I don't see that hotel listed on some of the maps, and I'm not positive now, but I think it was the Hotel Lancelot that we stayed at (the gated car park looks like it). It was clean, comfortable, walking distance to everything, and while after the Hotel Ala the breakfast wasn't overwhelming, but it was very good. (Look, I still DREAM about that urn of coffee on the Hotel Ala's breakfast buffet. After so many years it may have changed, but dreams are forever.)
There are many small boutique-like hotels in rome. Look at the Ripa area on the map too. That's cheaper but still caters to tourists. Just above Ripa there are a few also (one called "Exclusive Suite" caught my eye as it is between the Coliseum and the Pantheon and evidently next to the Circo Massimo.
Restaurants. We ate a lot at McDonalds or whatever pizza place that is available, but I do have one anti-recommendation. St Peter's and the Vatican Museum and the Sistine Chapel is amazing. We spent almost a full day there. We did join the group in the courtyard for the Mass and saw the Pope reciting it from his Papal Apartment windows. At lunch that day, we joined every tourist in Rome at that very colorful awning restaurant just outside the Vatican gates. Avoid it. It's way over-priced and the food was lackluster at best. Our guide for the Sistine Chapel that afternoon led us to a better recommendation just north of the Vatican. It breaks my heart that I cannot remember the name of that family-run restaurant. It was amazing. Ask your guide at the Vatican. They may get a cut from your meal price, but it will be better food and surroundings than that horrible tourist trap outside of the Vatican (I should have known better).
And another benefit of the smaller boutique hotels is that sometimes they have good food on-premise but sometimes they also know all the good restaurants in the area. If you're happy in your stay, you come back or others come back on your recommendation. They aren't stupid and they'll point you to something good. (I really did like the Hotel Lancelot.)
I'm out of time, but I will say this last thing. The Castel Sant'Angelo is a bust. It looks pretty, but unless you want a peon to the socialist struggle, it's pretty bare in there. The river view is nice. I was ready to leave in less than 10 minutes after the river view. Least favorite attraction.
And that hop-on, hop-off bus is worth every penny and a treasure.
