Thank you for your query,
It is looking like Diaper dermatitis
whcih may be due to prolonges wear of diaper or may be a fungal(candidal)infection which is common in children due persistent presence of wet areas.
You can apply zinc oxide
based creams which are available OTC and the area should be wiped gently after cleaning with water and should be allowed to dry and then apply the cream.
If it is due to candidal infection antifungal topical application is required so consult your Pediatrician
for examination and treatment.