I would have a really good look at all the costs involved in you returning to work. Not only childcare, but transport (for you and the kids), possibly extra food (as in lunches at work), wardrobe/uniform.
There's NO WAY I would be going back to work for $50. If you factor in the extra petrol you need, you probably don't earn anything. Even after the first 3 months, you'd basically be getting $325 (the $50 + half of $550) a week if I read you right. So that means you'd be on $8.13/hour. It really depends on how many other costs will be involved in your return to work. And how much you want to go back to work. And of course how much you need the extra money. Cause not only will you be working 40/week, you'll also still have to do your chores. So this could be majorly stressful and take further time away from your kids.
But it sure sounds like going back to work part time is not going to be worth your while. I would look at getting some weekend or evening work instead. Or even something you could do from home. Unless you love your old job so much that you want to go back to it.

I really take my hat off to you working mums out there, I don't know how you do it.

Saša