That sounds like a feature request! The feature would be something like: on switching world storage, give the user the option to copy/export worlds from the previous storage to the newly selected one.
I just found a solution to this on a Microsoft support forum (link below). It worked for me and at least one other user.
The solution, in a nutshell, was to uninstall the current version, find an older version somewhere (I used the SAI app to copy it from another device of mine, but others have used the aptiod store) and install that. The older version will allow you to log in to Xbox live. Once successfully logged in, go to the Google app store and 'update' the Minecraft app, and it will continue to work.
Obviously this is a kludge, and I don't know what would happen if I tried to switch accounts once updated (my guess would be another crash) but this is working for now.
Tip: switch the world storage option from internal to external to ensure that worlds persist throughout uninstallations and réinstallations.
Good luck. I hope it's properly fixed soon.
Same problem here with my fully updated and freshly wiped Samsung S7 running Android 8.0.0 (most recent available for the S7).
Minecraft runs ok if not signed in. As soon as I try to sign in it crashes. Beta works fine, but obviously no good for multiplayer or realms play.
Reverting from Beta doesn't fix the bug. Installing Xbox before or after doesn't resolve the bug. Installing minecraft earth doesn't resolve the bug either (though minecraft earth sign in works just fine).
Especially frustrating as i just bought this phone second hand specifically so my kid would have a device that can run minecraft.
@Gemma Phillips- as is becoming apparent from the various hack and tips on this thread, the issue really seems to be with new installs of the app on an Android 8.0 device. Your experience with your children's mediapads seems to re-enforce this notion. So I think the solution that worked for me may be appropriate for you too.
As I have Minecraft installed on several devices in my house, I was able to copy a less recent version from another device (my Nvidia Shield TV - not officially supported, so not automatically updated via the Google Play store) using the Split APKs Installer (SAI) app (available in the Play store - rooting not required). SAI clones all the data related to the app, so perhaps it's not relevant that the version I copied was older - maybe the important thing is that it had been successfully logged in as some point, I don't know.
Regardless, I believe that if you copy the version on your daughter's tablet to your son's you will be able to log into your son's account, and then, if necessary, update the app while still remaining logged in.
I would love to know if you are successful.