Absolutely the Freedom Trail, certainly walkable from Back Bay, or you can augment the walking by taking the T without resorting to taxi's which cost far more.
If your daughter has read Make Way for Ducklings then you should include the Boston Commons/Boston Public Garden.
Within Back Bay, Trinity Church offers many possibilities photographically.
There are motorized tours that are fairly reasonable too, that take in things like the USS Constitution and Navy Yard, Bunker Hill, etc. that aren't quite so walkable from Back Bay.
Here's a link that might be helpful; http://www.bostonbyfoot.org/