We got Immy a 12" one off Ebay back in September (she was 4 in the November). She is now outgrowing it (she is a tall girly). However, when we had tried her out on a variety of bikes she found the 14" a bit awkward, so we thought as a starter she would feel more comfortable with the smaller one. We weren't too bothered as it was 2nd hand and also it will get passed onto Isla.
Umm, so yes that wasn't very helpful! Could you get her a second hand 12" to get her started off, and then spend more on a 14" in 6/12 months time? Immy's bike was in really good condition, pink with tassles, etc and she loves it. They don't know it's not brand new! x