Rashes on the thighs are due few reasons. The number one reason is due to napkins. Using napkins with rough covers for a prolonged period will create rashes. Even soft napkins will lead to rashes if they are worn for the entire day. It is good to change the napkins once in 6 hours at least.
The unchanged napkins may also lead to cancer. 
Infections can lead to rashes too. Its is advisable to cleanse the intimate area using the intimate washes. Try the washes from: //shycart.com/productdescription.aspx?k=173

For more infections in the intimate part, use the 18 again cream: //shycart.com/productcategory.aspx?k=103

If the rashes are too bad then its advisable to visit doctor.