The best I could come up with is a separate instance of Pihole that any device my kids use is linked to.

It’s a little clunky, but you can do this with one Pi-Hole instance by using the Groups feature. In the “Groups” tab take a group for your default Pi-Hole settings (or just use the already included Default group), and then make a separate group for the additional blocked domains for your children’s devices (for purposes here we’ll refer to this group as “Child”). In your Lists tab, choose which Group each list should be applied to (or choose the group it should be applied to while adding the entry). In your Clients tab use the drop down menu to choose and assign devices to Groups, put all your devices in the Default group and put all your children’s devices in both the Default Group and the Child Group. This way your devices will have the default blocklists and your children’s will have the default plus the additional blocklists aimed to protect them specifically.