What about one of the mini ones like this
http://www.ebay.co.uk/itm/New-Iwata-...item3f06a1ad81
However I agree with Ian I think it will be down to the way that the brush is being held and there are a couple of different methods, I prefer a two handed method, I support the airbrush with my left hand and and you the right hand for trigger control.
similar to this picture
http://www.google.co.uk/imgres?q=hol...t:429,r:0,s:18
If shes using a one handed method then maybe worth her trying to use two hands, obviously this means you need another way of holding the bodies.
Hope that helps.