Police say 19-year-old Anthony Sifuentes was in the area of Barstow and Pollasky just before 8:30 p.m. Wednesday.
They say he pointed a gun at a man walking by and then ripped a necklace from around his neck.
Officers say Sifuentes also stole two bracelets before walking into a nearby apartment complex.
Investigators say Sifuentes eventually walked out of one of the units with a toddler in his arms.
Officers arrested him and returned the child to the mother unharmed.
Investigators say they located one of the victim's bracelets and found the gun on the roof.