I definitely use "Ms." when referring to adult women while speaking to children. That's how I introduce adult women to Miette - "This is Ms. Collins." I don't give her permission to refer to any adult by their given name unless that adult invites her to do it. And I introduce myself to other children as "Ms. Crawford" unless we have a fairly familiar relationship; then they call me Miss Heather. (My Girl Scouts call me Miss Heather, for instance.)

"Miss + First Name" is appropriate, I feel, for female children. (Though I do not say "Master + First Name" for male children. That just sounds dumb.) "Miss + First Name" for adult women is something that connotes a close relationship, a familiar relationship.