In all honesty (my opinion) the use of an amp with portable headphones is rather cumbersome and unwieldy in use, the gain in sound quality is minimal unless you are sat critically listening and it is far better to use a suitable headphone that can be driven sufficiently from the headphone jack output, she’s 14 yr’s old and prob listens to downloads that are sub 320kbps so i doubt she’s going to notice the difference nor will she be posting critical reviews on Head-fi like the spotty music geeks who believe the marketing.
FWIW I used to have quite an expensive collection of headphone amps/micro DAC’s from the likes of Ray Samuels, Headroom, Fostex etc..etc and i sold them all as they were a pain to carry and unless i was sitting down in a quiet environment then they made sod-all noticeable difference.
I have an Iphone 5s and these days i only use/own Sennheiser Amperiors headphones (good for portable equipment) , Klipsch X10i canal phones and i have a pair of Etymotic ERP-4 canal phones as well.
I also have plantronics bluetooth in ear but audibly they are really very poor, and a crap battery life so i haven’t used them since i bought them – thought they’d be good for the bike but nope – they’re shite.