How do I unadopt my children? (preferably without killing them)
Solution 1:
There is a Mod called Skyrim Unadoption Mod.
I haven’t tried it yet.
Solution 2:
There are 2 ways to unadopt children.
Unadoption Mod
Download here
If, like me, as soon as you installed Hearthfires you went to the Orphanage and adopted two children, before realizing that there was a poor homeless girl trying to sell flowers under the snow storms of Windhelm just waiting for you to adopt her, or if you simply want to get rid of you children, then this mod is for you.
The mod gives you a power (that works like a shout) that will send back your children where they came from and you will be able to adopt new ones.
WARNINGS: - While this mod is active, do not try to adopt more than two children. - After unadopting your children, there's a small chance you might not be able to re-adopt them. So if you're thinking of re-adopting, make sure to save your game before unadopting and then you can save again only once the re-adoption worked. Here's a fix if re-adopting doesn't work. It does not seem to work every time but it's quick and worth trying: Open the console, click on the child, and enter "addfac XX04290 1", replacing "XX" by the load order number of your Hearthfires.esm (for instance 03 if Hearthfires is third in the load order).
The mod gives you a type of power to reset children's als to before they're adopted. According to the warning, you can only do this once since you can't readopt [bugged out, so don't worry].
Start the DLC all over again
By uninstalling, and reinstalling the DLC, all of the thing in the DLC should reset [yes, everything! Even those 3 land plots and manors too!]. You should be able to adopt the right children and make them move to your home.