Do meet my employers for brunch every couple of months and we love it!

Just wait OP. Maybe do something closer to the holidays and not on a weekend? Like a christmas dinner where you give her a present and bonus.

It is very nice of you and the answer will depend on the individual. I can see some nannies not having a problem with it, but I can see a lot who do not want their work lives to interfere with their personal time.

I would find that intrusive and probably say no thank you as politely as I could. Later in the relationship it would be different, but I would feel very uncomfortable trying to make sure your kids didn't feel neglected, my kids were on their best best BEST behavior, trying to find mutually agreeable small talk, etc. Once I was more bonded with my NK, had a more solid relationship with you and knew you better I might be open to it, depending on whether I thought you would like my spouse and kids vs. judging them and me.