Some girls are just wetter than others even at this age. My niece is one of them. I could very well be other secretions, or sweat as well.
Make sure she wears cotton undies all the time and if you have to, change them twice a day. Skirts instead of pants will help her dry out. Avoid polyester, lycra, and rayon blende shorts and slacks as well because they don't allow for proper drying during the day. Liners at this age can lead to more trouble than they are worth since they can encourage yeast infections and irritate skin. If the moisture is affecting her skin then mom can try different powder and detergent combinations until she finds a soft blend to protect the delicate skin. Highly decorated elastics that get wet can cause raw skin too.
Active kids can squish their bladders or "leak" when being tickled too so mom should really check what's going on here.
Keep her hydrated so if she does leak, then the smell is not as strong.