First thing in the morning is when the urine is most concentrated due to the lack of intake of water, and because the urine has not yet been voided. The chance of finding a drug in the urine increases.
Or
Take a urine sample at the time you feel your child is high, before they void their urine. If your child keeps urinating, the substances in their urine become dilute as urine is being produced and voided. This lowers the concentration of the drugs, & the chances of seeing the drug in the urine.