If your 7-month-old baby has developed rash on their private parts, you can try allowing some air to reach the area by leaving their diaper off for a while. Make sure to thoroughly dry the area after bath time and consider applying zinc oxide cream if there is redness or irritation accompanying the rash. If the rash does not improve by Monday, it would be best to seek advice from a doctor. If the rash is extensive, uncomfortable, or your baby has a fever, it is recommended to consult a medical professional.