I found the alien registration cards and the 1939 Register on ancestry.co.uk, since at the time I didn't subscribe to FindMyPast.   Anyone with a world-wide subscription to Ancestry  or, of course the UK version, should look for the register in the card catalogue (which is about to disappear). However, one must realise that minors, who might still be alive, were supposed to be blacked out on the Register, so only guesswork based on the adults in their lives will locate them. 
-Eva Lawrence
St Albans, UK.