The likelihood of her needing a transfusion without you being first able to inform them of her bloodgroup would be pretty slim, so I wouldn't get one. As to whether people read them in an emergency, maybe - if she was bleeding alot, I would be focussed on stopping that and not reading a bracelet. If she were having a reaction to something, I would read the bracelet to see if she was alergic.
Just so people are aware, even if you do inform them of a blood type, a blood bank will not issue type specific blood without either doing their own cross match, or having documented proof of your blood type. While it is good to know your blood type, it is not always essential.