Your minors can buy and sell stocks with your approval.
If your minor has login credentials, they will be able to go to their accounts and place trade orders just like you can do with your own account. However, they will need your approval after they place their order.
Here is how you can approve or decline your kids' trade requests:
Open the sidebar menu
Select your minors account
Tap on a stock under Trade Requests
Here you have options to Approve or Decline a trade request
You are all set