Only one KND operative per planet may be chosen to join the ranks of the Galactic KND, a decision which is made through an elaborate and highly secretive process.
During the final season of Codename: Kids Next Door, Numbuh 1 is chosen to be Earth's Galactic KND operative. At the end of Operation: I.N.T.E.R.V.I.E.W.S., he passes his final test and confronts Numbuh 74.239, upon which the existence of the Galactic KND is revealed to him and the viewer. But in the end of the movie, it is seen that Numbuh 1 returns to Earth.
In the spin-off trailer, it's implied that the Galactic KND are actually much more sinister than previously believed, being willing to actually destroy adults and planets that they deem as "infested" with adults.
This coincides with their cover- Splinter Cell - who was believed to want to eradicate the world of all adults instead of just the evil ones.
It also expands on their claims in Operation: I.N.T.E.R.V.I.E.W.S. that adulthood is a disease.