Perhaps Elsie was trying to make Kosel sound less "foreign", and used Castle? Who knows!
I was wondering if children weren't part of their lifestyle.... they didn't go to New York right away. The 1922 passenger list and both of their citizenship papers said they had been living in Paris, France. Ivan loved the Hollywood scene, and a few news-clips have him acting in plays. Perhaps they just wanted to be part of the rich and famous set, and children didn't "fit in".
They said on the citizenship records that they came on the ship Aquitania. You can see Elsie on the incoming passenger list, but on the UK outgoing list, she is crossed out (usually means they didn't board). Ivan isn't on either list, unless he is hiding under another name.