I would hope that anyone capable of conducting a hair test is aware of hair growth and can estimate when last use was? If they cannot I would assume you can contest if you think the test gives an inaccurate result.
That being said, I hope you realise you are skating on thin ice and stop pushing limits. I have seen the result of parental drug addiction on kids. It's horrific. I hope you are not bull shitting yourself. You might want to consider plying a very straight and responsible course. As far as I can see you have already lied and that ain't good.
Do you want to be a good parent? I am sure you do. Be straight with yourself.