M.P.
Sounds a lot like chafing. I can remember having this myself at this age, and I remember it being hard to walk sometimes it was so painful. Plain old petroleum jelly works very well, like Vasoline or Aquaphor. Keeps the chafing down and acts like a barrier. Also, I have a household of sensitive skin. You might think about changing laundry detergents to free and clear if you haven't already, and bypass the dryer sheets and fabric softener. I use vinegar in the wash as a fabric softener, and it works great (an is much better on that delicate skin).