Each Google account is set up to be managed by only one admin/owner. In case your child's account is already managed by the School administrator, you will not be able to manage/monitor that account using Mobicip extension at home unless the school admin allows third party extensions to be installed. You will need to discuss this with the specific School admin.
To overcome this, you can create a new personal account on your child's Chromebook for use while at home and add Mobicip extension to it. Please follow the instructions in this tutorial to setup Mobicip on this personal account.