You have the right to request to return part time and your boss must give it reasonable consideration but they can decline your request if the role doesn't suit part time (after they have considered it as an option of course).

I think the best approach would be to say that you are looking forward to returning but would like to return part time and state the reasons eg. Lack of childcare, wanting to balance work/life with a baby. Then say that you have given it a great deal of thought and provide some back up arguments on how you believe the role can be managed on a part time basis. Maybe provide some options on hours/days and weigh them up for your boss so they can see how it can work on a practical basis.