Unfortunately, stillbirths weren't registered by the GRO until 1927, but you might be lucky if you can narrow down the location where the infant could have been buried - parish registers recorded stillbirth burials, and for the Aston area these will be kept at the local archives office (http://www.warwickshire.gov.uk/countyrecordoffice
), or still at the individual churches. The records office might be able to help you with where local cemetery records are kept as well.
Children for Patrick and Yvonne - if you search the Ancestry GRO births index (post 1915) for the surname Griffiths and mother's maiden name Crowther, you'll end up with 20 matches, but none born in the London area, which is where I believe Patrick and Yvonne lived. However, they seem to have been fairly mobile as a couple, so any of these could be theirs within reason, which doesn't help very much really. One potential lead I found was this passenger list for arrivals in New York in 1939:http://search.ancestry.co.uk/Browse/view.aspx?dbid=7488&...
I hope this link works and you can see the image! :-)