If your granddaughters fall into the typical, say 5'4 to 5'8, or so, then you could be building them their "forever bikes", or getting a frame that could be upgraded in the future.

The Motobecane Grand Jubilee photo above was done while my niece was visiting at about age 9 or 10 (thus, a rather low seat). She'll now be turning 15 in a week, and the seat has been raised several times when she visits during the summer.

Unfortunately, I fear she could hit 6', and she could outgrow typical women's bikes.

Of course, when dealing with kids, there is always the risk of damage or loss of bikes, or losing interest in them over time.