Sorry to spoil the fun but I'd suggest going back a step. Why does this lass want to move onto the Low D?? (I assume by "Lass" you do actually mean "young and growing"??) Unless she's already "shit-hot" (if you'll excuse the expression) on the "ordinary" whistle then she'd be better off getting a good one of those and working on that a while longer. Then when she's grown a bit, AND is a really skillful whistle player, the size of the low D will be that bit less of a problem, and the skills will transfer across that bit more easily.

Of course, it might be that she already is THAT good - although I tend to suspect that if she were the question wouldn't be being asked; she'd just be getting on with finding her own style and mechanics for playing the thing.

G.