Have you considered one of the Residence Inn properties? They have larger rooms, a kitchenette, and a good free breakfast included. All of these amenities are typically great for family stays especially with young kids. I considered staying at one of them during my Boston trip last summer; however, since I wanted to burn IHG points I didn't stay at them. Residence Inn Boston Cambridge is in the same general area as The Boston Marriott Cambridge you are already considering.

--Jon