I got a message that a baby had died from collision. I didn't find the body ANYWHERE. But after a while I got another similar message. Now I found the body and figured out what happened.
In the skyscraper that I built, the first baby had suicided by jumping off the roof. Around 15 stories high, too. The reason? His parents got killed in a goblin ambush earlier, and he was miserable. Second one had also a similar reason.
And the bodies were on the prison building's roof, next to the skyscraper. That's why I didn't find them earlier. I checked it accidentally, and saw a pile of dwarf bones plus a rotting baby body next to them. Quickly built stairs to the prison's roof, and the rest of the dwarves took the babies, or what was left of them, to their graves.
Pic of the fall:
And another, two stories which happened at the same spot, second happened because I forgot about the first. These happened in my other aboveground fort, not the one pictured above:
1. I learned not to build floor tiles at the end of drawbridges. Especially when they're 16 Z-levels high on the top of a skyscraper. The floor collapsed immediately when it was built, causing it to crash straight through the roof of my prison, and the ensulting blast of dust and debris knocked three dwarves and a cat unconscious. The engraver who built the floor smashed a few seconds after onto the prison roof and exploded. And then his kid who was with him fell too, but SURVIVED somehow. He died a few seconds later on his bed due to the massive injuries sustained.
2: I built a floor at the same spot again because I wasn't thinking. This time the piece of floor fell NEXT to my prison, smashed through two god damn levels of earth and finally exploded in my largest food stockpile at Z-level -2 (0 being ground level), knocking half of my fort (they were all eating/drinking there) unconscious. Oh, and the guy who made the floor also fell down, but missed the gaping hole the floor made and smashed next to it.