Corn on Foot

Sounds like it might be a plantar wart.
If so, get the acid disks and apply a new O. after every bath & cover with a large band aid.
Our podiatrist wanted to put my son under & surgically remove his plantar wort, he said the acid patches wouldn't work, but I insisted we try them first. It's gone. Took about 2 weeks total.
And if it is a corn--treat the same way.
You can buy a 2" x 3" removal patch (that you cut to size) at the pharmacy for about a buck. You won't need more than two.

It really sounds like a wart.
The wart removers you'll find int in the pharmacy work very well.
I've used both the acid patch removers and the freeze them off removers.

That is a wart not a corn. I used to get them there all the time.

I agree that it is likely a Plantar wart. They are contagious, so your child should wear shoes, slippers or flip-flops and not share shoes or socks till it's gone.
Some people seem to swear by the OTC cures, but I never found that they worked and ended up at the doctor anyway. And it's a one trip cure with the doc as opposed to weeks of patching or drops. But that's just my take.

Sounds like a wart. Try compound W or the Acid pads and go from there. If there is no improvement within the week she should she the pediatrician.