Sara Molina, Tekashi's baby mama, tells TMZ ... she chose not to allow Tekashi to see their child this Christmas because he's been completely out of the picture, despite promising to get back in 5-year-old Saraiyah's life.

6ix9ine's baby mama says he told her back in August he wanted to create a genuine relationship with Saraiyah and was going to be more involved and present ... but Sara says it never happened.

Sara claims Tekashi told her he would support her financially and help with their daughter's expenses ... but she says it turned out to be an empty promise.

69's baby mama even says she gave him an "open door" policy where he could come see Saraiyah whenever and wherever he wanted ... but even that didn't pan out. Sara says early September was the last time Tekashi sent any money or saw his daughter.

What's more, Sara claims Tekashi missed his daughter's 5th birthday back in October, and she says he didn't send a present or even place a phone call. Sara says it was really offensive ... she felt it was the one day he could have really redeemed himself.

It's kinda sad ... Sara says her daughter wasn't even fazed by 6ix9ine's absence on her birthday, and she says the kid recently began referring to Tekashi as a "friend" ... even though the girl knows he's the father.