Stay in the North End which is on the waterfront. The North End is walkable to everything. If you stay at Marriott Long Wharf on Commercial Street (link below), aquarium is right thru the back door of hotel, great for your 6 yo. Restaurants galore on Commercial Street and be sure to walk to Hanover Street for true Italian salumerias, coffee shops, bakeries (Modern and Mikes).
If hotel budget isn't limited, stay at Battery Wharf on Commercial & Battery Street (link below). Get a room with outdoor space. This hotel is at the other end of Commercial Street from Marriott Long Wharf. In between the two hotels which are on the waterfront (Boston Harbor) is Christopher Columbus park and walkway. You can walk to Faneuil Hall (an outdoor/indoor market) filled with everything to delight a 6 year old. Tons of entertainment and music, shopping, cafes and vendors. Boston is a walkable city. You can easily get to Newbury Street, and Boston Commons (take DC on the Swan boats).
The best view for fireworks is the North End. If you stay at Battery Wharf, you're right there! From Long Wharf hotel, you're also right there on the other end of Commercial. Trust me on this...I lived on Battery Street and me, DH and DS would lay in bed, turn off lights, open up a wall of windows and giggle during the 4th fireworks. We miss it so much. Have fun!
https://www.marriott.com/hotels/travel/boslw-boston-marriott-long-wharf/
https://www.batterywharfhotelboston.com