Rohan Marley -- I'll Be Mr. Mom ... When Lauryn Hill Goes to Prison

Lauryn Hill’s 5-time baby daddy Rohan Marley says he plans to STEP UP his parenting during Lauryn's upcoming 3-month stint in federal prison ... telling TMZ, "I will be there for my kids."

Hill and Marley dated for 13 years -- but never married -- and since they split in 2009, Lauryn has done the lion's share of the child-rearing. The kids range between the ages of 5 and 15.

Rohan tells us he's always been involved in his children's lives ... but says, "I will be there even more while she is dealing with this … to make sure that everything is fine.”

We're told ... Lauryn and Rohan agreed that the kids will primarily stay with family members, with Rohan getting involved as much as he can.