its called pronation and can be from a number of things, my son has it and in his case it is a combo of a week arch and actually pushing his heals out. we saw it right way when he stood up and walked (for him he did both of these thing very close together and a tad on the late side. we asked the ped and at first we took a wait and see as is usually suggested. the toddler arch is covered with a fat pad and is hard to see, that does not mean it is flat, its just "filled in" and it can take till 3 or 4 before that fat is totally gone. that being said it can also mask a flat arch and a good pediatric orthopedists can usually tell the deference.
anyway my son just turned 2 and we had him seen again because while it got better a little in the first few months we stabilized his walking, it stop getting better and i see it trip him up when he started to wear shoes and cause bad calluses and some deformation on his big toes that he now uses to counter balance.
two things we heard, first of all it does usually self correct to a large degree and most will talk a wait and see till at least 4ish. and that while there is some disagreement as to is shoe inserts help, most say they dont actually change the foot but rather just hold it a better why when you are actually wearing them. one thing they all looked for and were happy to find in my son was that everything was super flexible and cold be easily moved into the right alignment, if this is not the case, i get the feeling it is not a self correction issue and so this is important thing to know.
that being said it depends a lot of the severity, so you should have her seen by the right people.
in my case we in combo with a few doctors and physical therapists decided to do inserts, not because we think it will magically correct the main issue but that he complains about his big toes and we want to reduce the amount of secondary damage that is done to his toes or other joints that might be getting disproportionate pressure placed on them. so for his toes, ankles and knees we are addressing his feet, hope that makes sense.
i would have her seen and talk to a few folks if the first one is JUST in the wait and see camp