Gordon Farquhar

@ryan ashton - thank you!  Like Joe I was unaware that storage permissions needed to be updated under the child's account.  Having just tried it now, issue is resolved.  I have a VERY happy 7 year old son again.  Thanks all.

The workaround suggested above unfortunately did not fix the issue for my son's fire tablet.  Both permissions were already ticked.  I unticked them, restarted tablet and then re-ticked the permissions options but issue still persists.  As Ryan mentioned, it seems the issue does not occur on older fire tablets as my other son's fire 7 has no issue logging in with his microsoft account.