On your Windows computer, leave yourself as an administrator. Then give each kid there own, normal (not adm), account. This way, each has access only to there own accounts.

Firefox can create separate user accounts for each user on the system.

http://windows.microsoft.com/en-us/windows-10/create-a-local-user-account-in-windows-10

Just don't enter a password when setting up a local user account. And if you do want the new user account to sign in with a password, just enter and verify the password.

Not really a Firefox support issue, though.