Separate Minecraft accounts for 2 kids on 2 iPads, but in Family Sharing

My twins have been sharing one Minecraft game and its one avatar for a couple of years. Now that they're 9 and stuck at home thanks to COVID-19, we bought them each a new iPad. Now we want to keep the existing Minecraft world that they've worked on so much, but also we want to buy another "instance" or download of Minecraft with the goal of both kids having their own avatar, so they can play together as two separate avatars / characters.

I set them up with Apple IDs for children under 13, which appear as part of Family Sharing, of which I'm the Organizer. That seemed like the right thing to do in general, but for Minecraft it seems to be ... undermining us (sorry about the pun).

When I try to buy a new copy of Minecraft (so we can create a second avatar, remember), the App Store says, "A family member has already purchased this item. To download it again for free, select OK." I thought, "I don't think this will work to give us two separate avatars, but let's try it, just to confirm." So, I download the Minecraft app to dear daughter's iPad. However, I don't even get that far: When I try to launch the just-downloaded Minecraft app, I get "This app is no longer shared with you. To use it, you must buy it from the App Store." ["View in App Store"]. Which I thought I just did! So, I can delete it and download it again, but the same error messages repeat, in an endless loop.

Does anyone have any suggestions about what I should try, so my son and my daughter can each have their own Minecraft avatars? The next step, by the way, is to let the new avatar enter the world of the old avatar by going through a Microsoft XBox Live account which we set up with guidance from a gamer college kid. That way, I believe, both my children can play together in a shared Minecraft world and keep working on the huge virtual houses and farms they have built there.

Thanks in advance for your help!