The guardians Grandpa, Nanna, Mom and Bro were created in the laboratory by John using their own genetic material sourced from future versions of themselves.
Grandpa and Nanna's DNA merged and created John and Jade. Likewise, Mom and Bro's merged to create Dave and Rose. So now these 8 babies are hanging around in the lab.
During the reckoning, each baby is sent through a portal to Earth on a meteor. Grandpa and Nanna land within about a week of each other 1910 in around the same spot. They are taken in by Sassacre and his wife Betty Crocker and his dog Halley. Sassacre dies shortly after and the two babies are raised as siblings.
13 years later, Grandpa leaves (apparently with the dog, who by most dog ages should have died by now so it's probably Bec) to become renowned explorer and billionare.
Now for a bit of what we haven't seen yet. Nanna grows up and takes the name Egbert presumably from marrying some guy. Together they have Dad.
Later, probably sometime in the 70s another meteor lands with Mom on it. Either around the same time or later still, another meteor with Bro arrives. We don't know how they grow up yet.
Fast forward to 1995. The meteor carrying Jade hits the Betty Crocker factory (owned by Grandpa) in John's future neighbourhood. Grandpa takes the baby to the island in the pacific to grow up.
Days later, meteors carrying Rose and Dave hit their future locales. Mom and Bro are there respectively to adopt them. Months later, April 13, 1996, another meteor carrying baby John hits his neighbourhood, tragically killing Nanna, but Dad is there to take him in.
The kids then grow up and enter Sburb and John reaches the lab where he sets all of this in motion.