I agree re the tax, but if you put it that way she is also not receiving superannuation, loading etc. If I do work for DP (on the books) it's not even worth doing for less than say $40 per hour because I could be receiving much greater entitlements doing more hours at my regular employment.
Personally, I would raise it to $200 if you feel she is a good carer for your children. I was asked to drop my friend's daughter to school daily for $50 per fortnight and everyone jumped on me and said I should because "you don't have to do anything"... caring for other's children to me is much more than just playing, I am in charge of their safety for a period of the day and it is taking away from my family and/or leisure time. If it's "not doing much" then I take back what I said about SAHM being a hard job.