Two Possibilities….

[Unregistered-Dodgeman]

Buy the kids a ferret. It will keep them entertained, and they can train it to go into the heating ducts to retrieve the toys!
On the real side, how about the vacuum cleaner? You could push the hose down into the duct, and hopefully the toys will stick to the end of the hose, and you can pull it back out.
I don’t know about the 2nd floor, I would assume the ducts drop all the way to the basement from there.
How are they getting the toys in there? aren’t the registers screwed on? If not, it sounds like another project!
Good Luck!
Dodgeman