WE did that once, though our helper was older. Just make sure you have everything outlined ahead of time. If she's under the impression that she's just helping for a few hours here and there and thinks she gets to go off on her own the rest of the time, she might be frustrated...and you might too.
Make sure she clearly knows what's expected of her. Will you all go on rides together and take turns with CS? Explain how that will work to her. This is especially important because she's 15...and make sure her mom understands everything.
Maybe have it broken down - the costs that you will be incurring for her - so she can see what she's really getting "paid." That might help. And you might see that you want to give her a little extra for some souvenirs - depending on the costs for everything else. IN that, remember the hotel costs...would you stay in the one bedroom suite anyway, or are you staying there because she's coming and you need the space? If you're doing it because of her, remember to show the extra hotel cost as money she's being "paid" - if not, don't include it.
Cell phones are a MUST. If she needs to get a hold of you for any reason, she needs to be able to - though there are places where you may not get cell reception.
My biggest thing is to make sure you both have the same expectations - otherwise it could get ugly. If you can't tell, we had that happen. Our helper got upset that we wanted time to ourselves - though we "thought" we had made that clear ahead of time (so sometimes I guess you really don't know if they're understanding it). I think we had differing expectations and they didn't get communicated. However, a fifteen year old maybe less picky than a 25 year old?
Even though she's your helper, make sure to include her in some of the decision making. "Alright, we've gone on all the Fantasyland rides, 'helper', what ride would you like to try next?"
Hope that helps!
