When I was a kid I read and re-read a YA novel, set in the Andes, about siblings who crawl through a tunnel and find a valley where dinosaurs and cave people co-exist. I believe it was published in the late '50's or early '60's. Obviously pretty much taken whole cloth from Doyle's "The Lost World," and, in retrospect it was a badly plotted book, but I loved it back then. Still, I'd like to have the name again.
Two parts I remember are 1) a boy crawling through the tunnel in the beginning and finding a freshly killed archaeopteryx, and 2) the youngest toddling around saying "pleasure's yours" instead of plesiosaurus.
Any help will be greatly appreciated. Thanks.