I guess you have not been following this long enough or just don't understand because THE WHOLE THING IS LIKE THIS!
John did the ectobiology dance and sent all of the babies of every major Earth character back in time, one of which would grow up and become John who would...
Dave sent Terezi all those boondollars because that's how she found out about the human kids, so at a certain point she got him to send her that amount of boondollars so that she would have discovered the kids which lead to....
Dave eventually became the Dave that would look around the corner and give his other temporal self a thumbs up so that he would become the one that looks around the corner and...
...and so on. It's implied that you're not supposed to think about any of this too hard.
I don't think you read my post.
Yes, that explains why THE KIDS exist, but why do their dream selves exist?
It is the same universe! That means that there is a physical copy of them out in space somewhere,
not in a dream or something.
Also, this web comic was meant to be questioned and scrutinized. That is why there are so many details.
Hussie would have us believe that there are no major plot holes of any kind.
And for the most part that would be the case.
Lordnincompoop's answer was dead on though, except that sort of stretches the use of the word "now".
EDIT:
It is also implied that their dream-selves have been around for most of their childhood.
Since Jade was apparently getting her secret info from her dreams.
EDIT:
Also, I've been posting on this thread since reply 2947 out of 16607.
You must not have been reading this thread for very long.
EDIT:
Sorry for overreact. I'm tired.