Maybe something along the lines of 'when a mummy and daddy love each other and decide they would like to have a child, the daddy puts a seed in the mummy's tummy and a baby starts to grow'.
Not sure how to go about it if she wants to know the exact way he puts it in there though!
LOL Thing is she asks questions all the time! So I wouldn't be surprised if she did ask how he puts a seed in there.

As for "special adult cuddles that sometimes makes a baby" I'm not sure if she would ask for more details. She has a need to know about absolutely everything. Which is great, until this question. But this might be my best option.