Maid crisis: ongoing talking back and attitude demonstrations

Ok ladies, I need help... seems like I'm incapable of handling maids!!
They (we have our second maid from an agency) just keep talking back to me and demonstrating attitude. I mean, as long as I am super friendly and mega polite - no issues. If I am not - I'm getting cr*p. Latest incident:
Today, we took kids to a playground. My daughter was on swings, I was watching her. My 4 yrs old son was standing nearby, the maid was watching him. Next thing I see - my son gets hit by another swing! I jump to him, take him and tell her - why aren't you watching??, to which she responds, "I told him not to run". I take him to a bench, she comes up with my daughter, I tell her: "He is four years old, you can't just [i]tell[/i] him not to run, you should make sure these things don't happen!", to which she puts on a sulky face and responds in a raised voice: "What am I supposed to do?? Jump under the swings??"
I mean - seriously? WTF?? Was I agitated? Yes - my son just got hit by a swing on her watch! Does this justify that kind of answer and that kind of tone? Wouldn't "I'm so sorry, I hope he's ok!" be more appropriate?? At least that's what I would have said! Oh - and it was just yesterday when we had a long discussion of what is acceptable and what is not in terms of tone and attitude, just after another incident of same nature!
I need your advice here ladies as I am not sure what to do next. I can't send her back to the agency (we don't get any more replacements) and I don't really want to as she is - generally - brilliant with kids and keeps the house clean and tidy. Plus, I am sure the problem lies with me and not her because I had same issues with our first maid. I'm just not sure where it comes from, and why. My suspicion - not tough enough, too friendly and too generous! So yes, whatever I said before about being nice and friendly to your maid - I am ready to take it back...
Anyways, I have two tasks at hand now:
1/ stop this attitude
2/ make sure the actions I take don't affect the way she is dealing with the kids (I don't need her to be smiley with me but she must stay friendly with them).
How do I do this??