One of them is 9...and she would probably be OK with this...the younger one probably isn't there yet. The problem is i'd have to stay in the house with the 9 year old knowing she's got ready on time waiting for the younger one and its not then fair on the 9 year old. Neither of them are old enough to make their own way to school its a touch too far.
You have an interesting approach though...is she ever late ? Do you stand there knowing she's going to be late and let her get on with it ?
Never been a need, she has always been ready on time. Though sometimes a quick "leaving soon" is needed to remind her.