Lead-based paint is a serious health hazard. It's especially dangerous for children. When they breathe in the dust or even put paint chips directly into their mouths, they can suffer from learning disabilities, IQ issues, mood swings, aggression issues and much more. Lead can stunt their development. While some of the signs only show up as the child gets older, exposure at a young age can have an impact for the rest of their life.
What you may wonder, then, is why lead paint was ever used. You know that people did not understand the negative impact of lead in previous generations. They didn't know it was toxic. Just look at the Romans, who used it in pipes and plates and cups. Only in recent decades have people really discovered just how bad it is.