# if a millstone were hung around his neck and he were thrown into the sea

"if they were to hang a millstone around his neck and throw him into the sea" or "if someone were to put a heavy stone around his neck and push him into the sea" 

# a millstone

This is a very large, heavy circular stone that people use for grinding wheat grain into flour. Alternate translation: "a heavy stone"

# these little ones

"these people whose faith is small"

# to stumble

"to sin"