Sharing music with my kids...

Up until now, I have had all my families devices synced up to the icloud and itunes match, so we can share the same calendar, photos, and contacts and music.

Recently I've had my kids sign into their iphones and macbooks with their own apple account/password, so I don't have to pay for all of their apps and music.

My question is this: if my kids have downloaded certain songs on their devices, that I want in my library, how can I get their song on my device, since they are signed into different accounts?

Thanks for any assistance.

Greenerimpact