What's the difference between a baby and a paper airplane One glides if you chuck it off a building
Why are you trying to say about babies?
Anyway, it's just an innocent joke about some dude chucking a baby off a building for fun.
The death of a child can be extremely traumatizing to parents and affect their whole lives. Making light by joking about throwing a baby of a building  can be hurtful, just like you would be hurt if people made a joke about something that caused you pain.