Whether this has a chance of working depends on the exact diameter of the narrow portions of the submerged route. My impression is that they were very narrow, perhaps too small for a rigid container containing a human?
https://ichef.bbci.co.uk/news/624/cpsprodpb/146FC/production...
I recall seeing one of a vertical cross section so you can see where the kids would need to go into the "submarine", but I can't find it again nor do I know how accurate that map is.
In any case, the above map has a few cutaways showing you the cross section size of the tunnel to human scale.
I think it might work. It helps that they are very young and small and athletic build (and now, unfortunately, really skinny due to not eating much food for the last couple weeks).
https://www.google.com/search?q=diesel+fuel+bladder&num=100&...