Children may believe "mental assent" in a make believe person called Santa Claus and that he travels the world in a sleigh pulled by reindeer delivering presents, but that's a far cry from believing in Santa Claus to save you from your sins and provide for you eternal life. To believe in Jesus (unto salvation) goes beyond mere "mental assent" belief in the existence of and in historical facts about Jesus.
It's not enough to believe in our head that the death, burial and resurrection of Jesus "happened." We must also believe in our heart by trusting in the death, burial and resurrection of Jesus (Christ's finished work of redemption) as the ALL-sufficient means of our salvation. (Romans 1:16; 3:24-28; 10:9,10)