I'm 5'3" and my errand-bike is a Trek Navigator 200 with the mixte frame. I didn't remember the frame size so I went down to check, but the sticker has fallen off. Hah. I want to say it's 15 inch, but wouldn't swear to it.

Personally I like the mixte frame, because I ride that bike to go out to dinner and to church, and sometimes I like to wear a skirt.

I'd say the flat bar is more comfortable for very low speed riding (like she'd be doing with her 6-year-old). I believe the 7100 is a very similar bike. I chose mine for its comfortable, neutral seating position, even though I felt really goofy buying a 24-speed bike to ride round trips of two to 20 miles in coastal Florida (and the shop stocks the bikes for the same reasons).