Best is Prostocks. But Minor account opening process is offline.
Fyers - seems to be another good option, everything is online. Within 2-3 days your account will be ready. Check with their support for what is possible with minor account.
Zerodha is useless in case of minor account, because you can't sell allotted shares from minor account, you need to transfer the shares to guardian account.
Note: Do not confuse Trading account with normal intra day trading, both terms have different meaning. A trading account is a must even in case of minor for selling the shares. Brokers like Prostocs and Fyers provide the demat+trading account for minors with some restrictions.