Bam Margera's Ex Claims He Gave Their Underaged Son a Cannabis Roll-On

Bam Margera's ex is asking a judge not to change their custody arrangement ... claiming Bam's been in and out of their son's life in recent months -- and alleging in the order that he gave their son a roll-on device from his cannabis line, "BAM THC."

Nikki Boyd filed the documents in court earlier this week in response to Margera asking a judge to grant joint legal custody of their 8-year-old son, Phoenix ... telling the court that Bam hasn't been as present a father in the last six months as he's led them to believe.

Nikki's order states that Margera had 48 weekend days during which he could spend up to 4 supervised hours with Phoenix ... but she says he only visited on 7 of the 48 weekend days ... and only once stayed the full 4 hours!

She also questions Margera's judgment about what is appropriate for their child ... alleging he gave their son a roll-on cannabis device, which he brought home. A roll-on for cannabis typically resembles a deodorant stick, BTW ... and is usually used for targeted physical relief.

Nikki also alleges Margera gave Phoenix a sticker that reads "WRONG ONE TO F*** WITH" -- the swear word is not censored on the actual sticker -- and tried to slip an unmonitored smartphone into his bag when he was only 7.

The document asks to keep the custody agreement intact in order to provide Phoenix with much-needed stability at a critical age.

Nikki adds that Bam may be motivated to seek increased custody rights because of a recent child support order -- a story we broke -- rather than a desire to be a more involved dad.

As we reported, Bam told a judge last month he's ready for a larger role in his son's life -- asking for unsupervised visits and the chance to introduce Phoenix to his wife, Dannii Marie.

Bam's lawyer Peter Thompson tells TMZ ... Bam's focus has been, and continues to be, his relationship with Phoenix. He is a loving and devoted father who deeply cares about his son and the role he plays in Phoenix’s life.

He continues ... "The suggestion that Bam's request is motivated by anything other than his desire to strengthen his relationship with his son is inaccurate. Bam has been making significant efforts personally and professionally, and he believes it is important for Phoenix to have the benefit of a meaningful relationship with both parents.