My great nephews have both used a Geko 101 with no problems, and the youngest, who was about nine when he first used it, learnt it very quickly. The only disadvantage of the 101 is that there's no facility to download waypoints, so they have to be entered manually. That's not a problem for occasional caching, but if she wants to do a lot in a day that could be time-consuming. A 201 might be better.
I've no experience of the Etrex, so I can't comment on that one.
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)