Police say Sachs attacked an 8-year-old boy with a hunting knife inside a Dave and Buster's restaurant in Westbury, Long Island.
It happened around 8:30 Friday night.
Witnesses say the boy was stabbed while playing a game at the restaurant.
Authorities say the 23-year-old attacker picked the boy out at random and stabbed him five times in the back.
"The boy staggers away from this man back towards his mother and says that someone just hit or tapped him on the back and then he collapses," Lt. Kevin Smith from the Nassasu County Police Department said.
The boy, who is from North Hempstead, remains in the hospital with stab wounds and a punctured lung. Police say he is in stable condition.
The boy had been at the restaurant with his father.
Police wouldn't immediately say what provoked the attack.
At Sachs' arraignment, prosecutors said he confessed and even had a note on him Friday night describing how he intended to commit the crime.
Sachs' attorney says his client was on at least five different medications and needs psychiatric evaluation.
Dave and Buster's said in a statement that it is cooperating with investigators regarding the incident.