You need to complete each child's account opening process separately.
Once Zerodha verifies the account opening forms for your first child, you can visit signup.zerodha.com/minor and click on Open another minor account to start the process for your next child.
If the guardian for your next child differs from the previous one, the new guardian must visit signup.zerodha.com/minor and complete the account opening process.
For a video walkthrough on opening minor accounts, see Open a Zerodha account for your kids and invest in their future.