The answer here is b.
This is a tough one because the subject is so long. If you keep it simple in your mind, you'll realize that the subject is, when all is said and done, still "children." And the only word that's a natural fit after "children" in this sentence would be "told." Use "children" with the other choices and there's a good chance you'll hear the awkwardness. The phrase that follows children and complicates things forms a nonessential modifying clause. Below find some practice with that grammatical form.