Grandparents having grandkids stay over

That was the way I understood how the invite worked. The kids, babies, infants would have to be at the grandparent's home and after their arrival, I would need to save the game then go to Manage Worlds to play the vacationing parents. It'll be interesting to see how it all plays out. I was curious as to how that could be done because with two sets of grandparents per Sim baby in this save, my new parents (once the babies arrive) can look forward to a lot of free babysitting. I play with aging off on a rotation through many households, and I wander in and out of their lives in no particular order. But for some reason, perhaps happy memories of spending weekends with my grandparents when I was a kid, when I saw this was a new feature in GT, I was thrilled. Now I can't wait to get to that point in my current save. Thank you for making the process more clear. :)