Dehydrated 2 month old?

I am fearing that my 2 month old is dehydrated when she has to stay home with dad or go to daycare and won't drink much from a bottle.

We're only a few days in, and she is exclusively breastfed so I know there's a period of adjustment, but she is only taking about 8-9 ounces in a 10 hour stretch while I'm away. As soon as I get home, she wants to nurse and nurse for a long time. I noticed her front fontanel is depressed quite a lot too when I get home.

What can we do to help her take more breastmilk from a bottle and not get dehydrated?

I don't know exactly how much she weighs now, but she is over 10 pounds...

Am I worrying for nothing?