if you're in a situation where working additional hours isn't that you want isn't going to satisfy your employers needs, you have to be upfront with him. essentially, tell him you can swing the extra hours you have offered, but any more than that, you'll have to put your child in day care, and you'll end up losing money and you can't afford to do that. don't mention the second job - it's none of your employers business! he either takes what you can do, or finds another employee. if he's looking at adding another FT employee though, i doubt what you're offering will be enough...

as to centrelink, all you have to do is let them know your income, and your payments will be altered accordingly - not much else you can do there!