My 10 year old has exzema on her feet. What is the best thing to do for it?

Topical steroid, but. Mild eczema/dryness can be controlled with skin moisturizer etc...More severe cases need topical steroids, and depending on severity, different potency can be selected by your doc to treat it. But only mentioned in the feet--i am concerned he may have something else such as fungal infections (athlete's foot) etc..So should consult doc for diagnosis first. Wear good-breathing socks/shoes. Good luck.
Hydrocortisone oint. If it truly is eczema, 1% hydocortisone (hc) ointment may help. It's available without a prescription & usually work better than a cream-based hydrocortisone. It might not be strong enoiugh, sometimes a prescription-strength cortisone is necessary. Soaking in cool water may help, followed by a good moisturizer. It's possible the problem could be something else, like a fungus-could worsen with hc.