Hi. I'm also a not a mom, but I have a wife and a baby. I've done fairly well on the 9-5 schedule (except for a few nasty weeks around finals time) The amount you work and the grades you get are related, but only up to a certain point, and that point is different for everyone. You'll hear stories all across the board. Some people can just do all the reading and the classwork and they get good grades. Some people have to (or think they have to) do all the reading and classwork, along with reading supplements, tapes, study groups, swapping outlines, and recreating their own outlines over and over again. I have a friend who is a mother of 3 and she's in the top ten percent of our class. She seems to be able to get everything done from 9-5 during the week and on Saturdays.
Also, if you don't care about placing high in your class, law school can probably be pretty chill. You'd still be busy at least 5-8 hours a day during the week though.