If you find that your child's music is inaccessible after Premium installation, keep in mind it may take some time for all the music to re-load onto your child's device after the device reboots. Please give the device some time after it is restored to re-load all content.
After the device has fully rebooted, please tap the songs/music in Apple Music to trigger their availability.
If you still cannot access this music, it is very likely that this issue is a Family Sharing Apple account issue, unrelated to OurPact.
This Apple Support article outlines in detail how to access shared family content, including music.
If you still experience difficulty with this after following the steps outlined in this article, please contact Apple Support or firstname.lastname@example.org.