This is the best map I could find (linked off wikipedia):

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.