Hello,
I have been a nanny for 8 years, and in some ways i agree with anabala27's comment. Except i don't think you think we're stupid!
Just as much as you employers expect us to be honest, we expect the same from you. I know your nanny wasn't straightforward with you, but in this case i'd be the better person and just tell the truth. Yes i have looked at your facebook page, because you have been off sick too many times in this short period of time and i was suspicious. Ask her if she can explain, and do listen to her. (my current employer asked about my health on the first interview and i could tell her, that unless i'm so weak i can't get up, or infectious i'd go to work. In the past year i was sent home once with a tummybug, and went to work the next morning, she knows if i'm off i'm near death.
)
If you don't like her reasons, then you have two choices:
a. Give her notice, and tell her that if you notice any negative behavior towards the children, she's instantly fired.
b. Give her the notice and don't expect her to come to work. (pay or don't pay her for the period of time, depending on what the contract say. Ie.my contract says in case of loss of trust, misconduct whatever you call it i am dismissed and not paid, but(!) they have to have a very good reason)
When you are interviewing a new nanny, there are many things you have to think about regarding the children, but think of yourself too. Ask how many times the candidate has been off sick last year, any illnesses you should know about etc. It's all affecting your work too.
Hope the problem gets solved, and you don't think all nannies are unreliable.
PS. Anabala27 I am not "Just a nanny". An employee, yes, but part of the family too, who works hard to teach the children. I am there to comfort them and calm them like a big sister, entertain and help them grow. Don't let bad experiences belittle you (i had a lot too). You are important.