Apple really needs to let adult children within the family sharing plan pay for their own purchases!

Apple is not paying attention here. This is a user to user forum.

Go here if you want Apple to know what you think. (don't expect a reply) Product Feedback - Apple

They could always purchase iTunes gift cards, with their own money, and redeem them to their own accounts. Then purchases will be charged against their balance rather than being billed to your credit card.

No more getting billed for their purchased and we can still share music.

This is the entire design point of Family Sharing. A single payer, other than redeemed pre-paid iTunes gift cards.

These terms were fully disclosed before you started sharing.

Family Sharing - Apple Support

Apple has fewer issues getting paid and rewards the user with discounted pricing. Seems a fair trade.

sometimes they are wayyy too strict for my liking.

Feedback for Apple goes here >>> http://apple.com/feedback

wait...

Im about to cancel my apple music plan

If you're paying for the Apple Music plan, then why would there be purchases for Music? The whole idea of the Apple Music subscription is you don't have to pay for individual songs. They're able to listen to, and download without paying anything.

If they're buying through iTunes, then they're doing it wrong.

This is crazy when all I have to do is switch over to Spotify and problem solved

If the purchases that you're referring to aren't for music, then Spotify won't solve the problem.

If it is music, then they need to get the music from the Music app, in the Browse and/or 'For You' tabs, NOT the Store.

I hope this helps.

See this article Get an Apple Music Family subscription - Apple Support

Manage your Apple Music subscription on your iPhone, iPad, iPod touch, Mac, PC, or Apple TV - Apple Support

The iTunes Gift cards would let them pay for their own storage.