Will hotels in Italy allow 1 adult with 2 children in a 2 person room?

I would email the hotel directly, give them your info and ask if they can fit. I am traveling to Europe this summer and one of my rooms will be an adult with two teenagers. For Rome and Venice, I emailed the hotels direct and they gave me a triple room. Some told me can not accommodate because for fire laws and rooms pretty small. I did not have luck when looking at the Marriott's in Rome, Venice, Prague or Paris. Look for hotels that have triple rooms.

Spike, no, if specified for just 2 people, by law more than 2 can't be accommodated in that room. You need to book a family room or triple if room size is adequate and they can provide a rollaway.

As others have said, contact the hotel and say you're 3 people, they will give you choices. Don't arrive there with a reservation for 2 and an extra person, even if it's a child, hotels cannot let more people in, everyone has to be declared.

Not generally, no. Contact the hotel. Many will bring in a child’s bed and charge you a small fee. Depends on the age of the child. A baby is usually no problem, an 11 year old could be, and often 12+ are counted as adults so you couldn’t have 2 teens and a parent in one double room.

Some hotels will offer triple rooms, others will do the roll-in bed for children, and some just won’t allow 3 people at all. It’s not like the US where in most hotels 4 people in one room is never an issue. In Europe a double room means 2 people and for any more you need to contact the hotel and it may not be permitted.

Fellow Marriott person here...yes, always put 3 people on the reservation to make sure you don’t get turned away. Many hotel rooms that would be a “no” for 3 adults will be a “yes” for 1 adult, 2 kids. I think the computer algorithm for a lot of European Marriotts won’t allow an adult to be in a sofa bed, or when you put 3 people, it automatically tacks a rollaway to the reservation.

Most of the hotels have rooms for 2+ people. I do not know which Marriott website you are visiting, but if the website is not allowing that choice you can search the same hotel on booking and select that option. Avoid to state you are just 2 and then you are 3. Not a good way to introduce yourself... I'm 100% positive you'll find it. Lived in italy for a while...
Cheers

Consider spending your money locally? Family-run hotels in Italy are lovely, more family-friendly than big chains, and more flexible about configuration.

thank you everyone for your thoughtful advice! motivated by your responses, i did call the hotels directly and received favorable news. i am very grateful for your honest replies.