If you check on the first definition of Colin
on this site, you will find that Cailean
has been used as a Gaelic form of Columba
in Scotland. Columba
is the name which originally meant "dove"; the "peaceful" part would be someone's elaboration based on doves traditionally being symbols of peace, but is not really part of the original meaning.
The actual Gaelic meaning of Cailean
was "whelp." A whelp is a young dog or wolf. Obviously the interpretation "young child" comes from someone's reasonable idea that the term was originally being used metaphorically to refer to a young child, just as the modern English word "kid" was originally the name for a young goat but has also been extended to human children. But the literal meaning of Cailean
would be "young dog", not "young child."