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.
Skill Practice I
Skill Practice II