Child has adult icloud: want to set up parental controls

Second time posting this as I cannot find my previous post from earlier today.

I have a child who set up their school ipad before I could get access. They set up their icloud account as an adult. I want to set up parental controls. Here are the steps I have taken:

• I have changed the birthdate on the icloud account to well below 13.
• I have removed and added the child to our family, but it still shows them as an adult.
• I have rebooted the ipad.
• I cannot replace the icloud account: I believe the school's software (Mosley) is preventing that as it says I cannot log out of the icloud account.
• I have tried to set up screentime restrictions on their ipad, but it doesn't let me lock apps out at certain times (ie, homework time). It just limits total time.

We have bark installed but I have since discovered that all the child has to do is turn off VPN and that is rendered absolutely useless (works great on their samsung phone). My child has absolutely no self control over social media.

This is incredibly frustrating. My child is required to have an apple ipad for school, but this difficulty in setting up parental controls over the years is really turning me away from apple when it comes to purchasing devices for my kid. I would not have bought an apple if it had been up to me, and, as a lifelong apple user, that is really telling. Apple can absolutely do better than this. My child's school can: why can't I, the parent?!

Thanks in advance.