I think you're right hair remembers pot, but it seems very unlikely that anyone would waste good weed on a 7 year old???!!!
They can test hair...your hair is like a notebook with a list of your drug use.
I have done alot of work with drug testing but not hair.
I do know that they can trace thye drug and estimate the time it was used up to a few months .Notice how many guys have shaved heads there days? no accident
hair testing