I doubt,there's mod for that,because simmers were asking for It (including me) for as long as I can remember.
Anyways,this may not satisfy your wish,but here's what I'm doing to ,,put for adoption'' any unwanted child in hood:
-Create a ,,social worker'' or whole personel,lol.
-Place her on a large lot with a spacious house;
-Whenever you spot a family with over-excesive number of children-switch back to your ,,social worker'',visit that family (use MC or testingcheatsenabled true-select right mouse button-add to active family and VOILA! :D),bring kids to ,,orphanage''\remove parents from their family trees with MC (I am sure you are able to do so with It).
-If you decide that you want certain boy\girl to be adopted-do the same as above,just with the family of those willing to adopt.
It's neat when you actually can communicate with youngsters before bringing them home.