@PrinceMJ You need a trading account for buying and selling of shares i.e. trading of shares.
As per my knowledge, you can consider Prostocks for minor's demat and trading account.
In Zerodha, you require to open accounts for both minor & guardian and shares need to be transferred to guardian account for selling.
You can search in Google or contact their costumer support number to clarify your queries or send email to them.