Hi,This is Dr Nanda, often this happens with the children who are physically active .This is due to exessive sweatting giving rise to prickly heat
.Some times it can get in to bacterial infection also.
Suggetions;1)Dont allow the sweat to settle on the skin,wipe of the place regularly
2) she can use the anti bacterial talkum powder on a regular basis while on the ground and even after shower will help better. All the best ,happy playing to the young lady.