Actually the term "second cousin" does not have a concrete meaning. It can mean a true second cousin, or it can also mean a generation removed from a first cousin. The "once removed" terminology was introduced to compensate for this ambiguity.
Ah, I read "sons of first cousins" as "two people who are first cousins to each other each have a son". "A son of a first cousin" would be much less ambiguous. In fact, even knowing what you meant, I have a hard time reading the meaning you intended to convey.
Another clearer way to phrase it would be "sons of my first cousin are not my second cousins".
