Britney’s so upset, she’s determined to have Sean Preston, 2, and Jayden James, 1, drug-tested to confirm her suspicions. “She’s asked the court-appointed monitor and her lawyers if they can prove the kids have been exposed to environments in which drugs have been taken,” an insider says. They told her she should snip off a few strands of their hair right after they’ve returned from Kevin’s place and have them analyzed in a lab.

Indeed, “testing for secondhand marijuana use can be done using a lock of hair,” confirms addiction expert Marty Brenner. “The test must be done within a day or two after exposure. Secondhand smoke would definitely show up, and results usually come back in a couple of days.”

Since Brit, 26, submits to regular court-ordered drug tests herself, her legal team intends to call an emergency hearing if Sean and Jayden test positive for pot. “Kevin could lose custody,” says the insider.