Simple answer: $400.
But taxes aren't simple and baby sitting gigs can be particularly complicated.
The term "taxes taken out", typically means that the employer withholds taxes from your weekly pay. But to some it may mean" have to pay tax at tax time". The parents may not be willing to withhold tax and deal with the IRS paperwork.
Taxes on baby sitting income are handled differently, depending on whether you sit in the parent's home or yours. If you sit in the parent's home, taxes may not be due if you make less than $2100 (depending on your other income). If you sit in your home, the $400 figure applies. If you sit in your home, you are considered self employed and must pay your own social security and medicare tax (in the form of "self employment tax"), in addition to any income tax due.
As VolvoGirl's reply indicated, even though you may not be technically required to report less than $2100 of "household employee" income (if it's your only income), you may end up getting an IRS notice because the parents claimed the child care credit and reported your name and SS# to the IRS