Your situation sounds exactly like mine. I am on maternity leave and I have about four more weeks to go and I was going back and forth about how to tell my job I can't stay. I love the job, the benefits are great but if I went back, I'd be paying for childcare only and that does not make sense to me but as most of the other women have said, I'd go back and the first day you return, give your two week notice. That way you can still receive some pay for those two weeks to recover from the cost of childcare for those two weeks. And as in my case, you never know, if they really want you to stay, maybe they can work out a part time schedule with you and that way you would not have to quit totally.
At my job there are day and evening shifts so I could do part time in the evening after my husband gets home and that way we still would not have to pay for daycare and I could keep my benefits. Let me know how that turns out. How many weeks do you have to go before you have to go back?