What does it mean you dream of a child falling off a roof?

Your child must be scared of worried about something which is happening in reality to them.

Another response:

Dreams of falling frequently occur at the beginning of the sleep period (early in the nap or just after drifting off at night.) The "falling" sensation can be a literal interpretation of "falling asleep." This is often followed by a sharp jerk into wakefulness, suggesting that the mind is resisting sleep. It is a very common occurrence.

... Other falling dreams represent feelings of being out of control, or (as above) fears of "falling" into a scary situation. Small children cannot express their actual fears, but an older child might be able to find relief by associating the dream with something scary such as a bully at school or worry over an exam.