I have no AP experience, however I am a nanny who is currently working for a family that is TTC. When they interviewed me, with one toddler, they told me that they were hoping to "grow their family" but obviously had no idea of when that may happen. I was totally fine either way and told them so. I do have newborn experience so that wasn't an issue.
I would just mention that you are hoping to have a second at some point and that it may overlap with their employment with you. As for their experience, I would base it more on an overall fit with your family, if they needed some training perhaps you could do it during your maternity leave?
Good luck!