1921 census revealed that George Bertram was at a boarding school in Margate, but the school was bombed in the war so no records likely. George Bertam searches are in a different thread
https://www.rootschat.com/forum/index.php?topic=835085.01921 census did not come up with Bertha (born abt 1884 Germany), Oskar or Brunhilde Bertram so that was disappointing. Perhaps Bertha Kra(c)he/Bertram remarried or the whole family had left the country. Not that I can find a marriage other than a Bertha S Bertram marrying a Jean L Got or Gob in Bootle in 1920. But I can't find that couple in the 1921 census either. Perhaps not looking hard enough?