Questions about Minecraft IAP

What about if it's through play store on fire tablet? I've installed play store on this fire tablet.

Hello, James.

The Minecraft license purchase is per platform.

Example: if you purchase Minecraft on Android, your son can play on any Android device. So, if he wants to play on Xbox, a new purchase is required.

Yes, that's correct.
Despite fire tablets runs a custom Android version, Minecraft couldn't be downloaded on regular Android devices due to Minecraft was purchased on the Amazon Appstore, which is exclusive of Fire tablets.

Got it, thank you. It would primarily be on a fire tablet so that tells me it wouldn't carry over to an android device also.

Hello, James W. I'm Arturo, and I'll be glad to provide assistance with your inquiry today.

You can set purchases restrictions on your child account and enable “ask to buy”.

This can be done on the web, console and via the family app.
Below, I'm going to share how you can enable ask to buy on the web:

"To manage a member’s spending settings on the web:
Sign in to the Microsoft Family Safety site with your account.
Sign in to your Microsoft account
Under the member whose account you want to manage, select More options > Spending.
On this page, you can select the toggle switch under Credit cards for Require approval for every purchase.
Under Spending settings, you can also select the toggle switch for Spending notifications if you want to be notified whenever the member makes a purchase.”

Reference link:
https://support.xbox.com/help/family-online-safety/child-accounts/approve-child-purchases

I hope you find this answer helpful to your inquiry. However, I'll remain attentive to any other one you may have.

Kind regards,

Arturo.