The Mellops Go Spelunking

Written and first published in English, apparently

This is the 5th of 5 Mellops titles, 1957 to 1963, in the lists of works at both English and French Wikipedia. For this one, French Wikipedia gives no French-language title.   The others may have been written or first published in English. Per English Wikipedia, "In 1957, the year after he moved to the U.S., Harper & Row published his first children's book, The Mellops Go Flying, and his second, The Mellops Go Diving for Treasure."

Data from amazon. Page count from a tp edition.