I thought it was normal for young kids to have flat feet? DD's ankles roll inwards too. The physio says they generally give it a bit longer before going for inserts, but to go bare foot or with soft-soled shoes for the mean time. But then someone I spoke to the other day - whose daughter also has hypermobile joints - said they were getting fitted for inserts soon and she's not even 2.5.
Perhaps your situation is different, though? I don't really know.