Family plan but children only access to their own passwords

Standard used in my previous deployments was for families with (smaller) kids only having very few passwords in the default shared family vault (but left it in for future use) and just create new vaults 'per child' in which the parents would participate. E.g.

• Default family vault: Guest WiFi Password
• Create 'Parents' vault: Add mom & dad
• Create 'Johns vault' for John, adding himself and mom & dad
• Create 'Sarahs vault' for Sarah, adding herself and mom & dad.

Building onto Tom's recommendation, you could have your child's permissions as read only to begin with. This would allow them to access and use their passwords and other items without accidentally editing them. (This could cause them to lose a OTP for example). Once they're familiar with the workings of 1Password, you can add the read/write permission back in.