In my opinion you don't have to have both a ring-bearer & a flower girl - I think you said the 13 year old boy has already been asked to be a ring bearer & is really excited about it. If he wants to do it then I reckon you should just let him do it, even if you can't find a flower girl. He could walk down the isle first by himself maybe - it would look fine.
As for the 15 year old - I reckon contact her asap and let her know that you want her to be involved and ask her how involved she would like to be - as someone said, maybe she would like to do a reading etc. And seriously, if she does want to be a flower girl I don't think there's anything wrong with that.
What I'm trying to get at is it's your wedding & therefore you can do whatever you like, regardless of 'the norm' etc. For example, at my wedding we had a woman standing on my DH's side - she is his best friend and there was never any question as to which side she would stand on. No-one even questioned it, in fact people thought it was a great idea.
Good luck! Hope you get it all worked out! Don't stress - on the day it will be amazing!