Thanks for posting your query.
The itching and a rash
in the natal cleft suggests infectious etiology.
In a child it can be scabies
or an infection with parasite enterobius
The best option is to get the child examined by a physician at your locality for a confirmatory diagnosis and follow the advise.
Hope I have answered your query.
If you have any questions I will be happy to answer.
Please close the query if you have no other questions.
Wish you a good health.