You're doing a great job hun. Simple honesty is the way to go.
I answer any question dd1 has this way, ie if im on the loo and its that time of the month she might say, mummy you're bleeding are you hurt? & I just say no hunny mummy isnt hurt! She doesnt ask anymore so I dont tell her anymore.

IfShe asks why the baby isnt ather dads just tell her that the baby lives with the mummy just like she does. I highly doubt she'll probe any further