How can I disconnect child's iPad from Family Sharing without an Apple device?

For starters, read Create an Apple Account for your child - Apple Support . If your child is a minor child they cannot have an Apple Account without Family Sharing being set up and monitored by an adult. If this is the case, you will have to delete the child's account and permanently lose everything associated with it. I am not being mean to you. I am an Apple products user like yourself and just know Apple's rules. From: How to delete your Apple ID account - How to delete your Apple Account - Apple Support

Section: Can I delete my child’s account?

If your child is above "child" age (13 in most places, but there are exceptions) they can have their own account. Making full use of an account can be difficult for really young teenagers since it may require the presence of a credit card for them to download apps. Hence Family Sharing makes sense even for teenagers unless you are willing to provide your child with unfettered access to your credit card.

Family Sharing typically requires the Organizer have access to an Apple device but it seems to be possible on an Android device (I just know what the article says): Use Family Sharing with Apple Music on Android - Apple Support