In short: prior/during Asgard's assault old Loki struck a deal with Mephisto and Hela. Mephisto would give Hela a land for her Hel for which Loki paid (in a mischievous way, of course) and her in reward allowed his soul to not be bound to go to Hel once he died as other Asgardian's do. After his death on Siege Thor missed his brother and since he learned Loki wasn't Hel bound he called him back like he did to the Asgardian's back in the JMS run beginning of Thor v3. But unlike everyone else, Loki returned as a child.
It seems old Loki got tired of being so predicable and arranged for his own rebirth as a child so that he would not be bound to all his baggage and old ways, and so as a proper god of mischief and chaos become unpredictable again.
kid Loki's first appearance thor 617
Journey Into Mystery and The Mighty Thor
kid Loki appears in these comics Both are really good.