****************SPOILERS*********************
The Beggar Woman and Toby are the only real innocents in the world of "Sweeney Todd"--and a lot of good it does them. The Beggar Woman, through her ravaged mind, has (I think), a vague memory of home and happiness, and she wants to get back to that world--ironically, she achieves her goal (she is reunited with her husband in their old home, with her daughter hidden nearby) at the cost of her life. Toby only wants to worship at Mrs. Lovett's feet, but his naivete about her (and about the world) destroys him.
Johanna may seem an image of purity and innocence, but she learns damn quick how to survive in this world (it's her fast thinking and trigger finger that saves her and Anthony from Fogg's Asylum, for instance). This is something that usually gets obscured in the bubbly-bimbo way most actresses play the role.
I ask in all honesty/What would life be?/Without a song and a dance, what are we?/So I say "Thank you for the music/For giving it to me."