Young children who witness violence can have acute or post-traumatic stress disorder. "The immediate reaction is shock and horror," he said.
After events like this, communities typically set up crisis centers in a church or other public place where people can seek professional and spiritual help.
Turn the television off, say experts, but answer your children's questions. Don't disregard an older sibling who is watching the news unfold and is worried. They need assurance, too, he said.
According to Beresin, young children may not have "discreet memories" of the event, but they can still have an emotional reaction, experiencing nightmares or, conversely, emotional numbing, said Beresin.
"Some kids shut down," he said. "They may actually turn off and not want to be hugged or cuddled -- that's a normal response. Some kids are clingy, and others will withdraw."
Kids can also regress in the aftermath of a traumatic event.
Parents should not force a child to open up, but "don't let them be alone," he said.
One way young children can work out problems are through reenactment. "They may be playing a game about shooting and dying, and parents should not stop that," said Beresin. "Let them do it."
Young children can also ask questions that don't directly relate to the event, according to Rahill Briggs, assistant professor of pediatrics at Montefiore Medical Center in New York City.
"They can ask directly or less directly about guns, or heaven or death or about a pet that died," she said.
In studies of 9/11 one of the findings -- not a surprising one -- after the terrorist attacks was that those who were most directly affected "suffered the most," according to Briggs. Coping with grief long-term depended on the cohesion of the child's family -- "how well the caregiving system responds to distress. When it is proactive, by definition the children do better."
"What was the most incredibly predictive five years out was how everyone was doing before the incident," said Briggs. "It is the same for mental health in general, those who are coping well in their lives before a trauma are the most likely to cope well afterwards -- even if they saw the towers fall."