I hate when a show explains something that should be common knowledge. There's no way these could be the first foals born to a different race.
When the episode aired there was a lot of talk about "Mrs. Cake is cheating." but that doesn't make any sense. If the father was a unicorn why would one baby be a pegasus or vice versa? Not to mention it's a kids show. Anyway...
Changing diapers with your mouth???
Why did they only ask the mane six to babysit? There must be many other choices. And of course Pinkie is a good choice. She worked hard at it. Magic babies are just too much.