The mother of Booster Gold's son, Rip Hunter, is unknown. She is implied to be Trixie, but I don't buy it. Booster was Trixie's boss, so it would be kind of weird for them to be in a relationship, and they haven't shown much romantic interest in each other before.

If we get into the boring genetics of it, as a child Rip had blonde hair and blue eyes. Trixie has red hair and green eyes, which would give her a 25% chance of having a blonde haired, blue eyed child with Booster. Also, Trixie does not enjoy time travel; I doubt she would want to spend her entire life time traveling.

Harley Quinn, however, has been in a romantic relationship with Booster recently. She has blonde hair and blue eyes, which gives her a 75% chance of having a blonde haired and blue eyed kid with Booster. I haven't read many comics where she time travels, but it seems like she isn't opposed to it. I also think her relationship with Booster could have been long term, in the comics where they're together he clearly cared about her as a person and trusted her. there was no reason for their relationship to end off-page.

But that's just my theory. If a better candidate for Rip's mom comes along, it'll probably be her, but for now these two are my best guesses