I suppose the kids are more malleable.
This is how it's appeared to me (numbered for ease in case anyone comments or corrections on any of these):
1) The skitters kidnap/capture a kid, attach a device to his/her back, and enslave him/her for some purpose.
2) The attached device seems to both control and become a part of the kid, I don't think parasitical per se, though.
3) It was unclear to me in one episode if the device was actually becoming covered with skin on a human kid, or if that was only on a skitter.
4) Some of the kids have a bit more autonomy; others are under more direct control of the Overlords.
5) At least some of the skitters have parental-ish feelings for the human kids.
6) At least some of the skitters are resisting the Overlords and are seeking humans for assistance in this. Whether just to free themselves or both groups is unclear.
7) At least some of the skitters are working in league with a third group of aliens, who appeared at the end of last season.
8) The Overlords are in conflict with this third group of aliens. Any relationship between this third group and the Overlords or skitters beyond this is unclear.
9) Whether the Overlords are the relative good guys compared to the third group is unclear, but the Overlords appear to be using Earth's resources for their own fight, ignoring the needs of the humans and willing to kill most of them.
10) This third group approached the humans for some purpose. I think this was with skitter help. They could be better than the Overlords, or just be using the humans.