People say visine causes false negatives. I'm guessing if you put a bit on the urine or something, mix it, then give it to them you should be fine (that sounded nasty). Well, I honestly don't know if visine really works. I've just read about it on erowid and even then they weren't sure either.

http://www.neonjoint.com/passing_a_drug_test/index.html

That site will help you. The kits you got should help too, but doing some things on the above site will also help. Green tea can do wonders.