I have no arches and am completely flat footed (I do an awesome footprint with paint!). My parents never picked it up or worried about it. I also have overpronation (I think that's what its called). At 43 its only ever affected me being able to stand still in one place for a long time - so I avoided working as a check out chick and instead was a waitress and ran around fine. I don't like to stand in queues either. I think just watch it over time and see how she develops.

Oh and I have never worn inserts or had any joint problems.