If a mother and father both have genital herpes, what are the chances of their new baby having it?

Manageable risks. If a mother delivers vaginally with her first genital outbreak, there is ~30% risk the baby could get it and have a terrible illness. If a recurrent outbreak, this drops to ~4% but suppression of outbreaks or c-section delivery before the membranes are open 4 hours can cancel the risk. Rarely, some women shed herpes without an outbreak and could pass it , baby sick around 2 wks.
Unfortunately high. Some mds recommend caesarean delivery since herpes can be dangerous to the fetus as he/she passes throught the brith passage.