Journey Into Mystery, Vol. 1 #100 (1963) 1st app. Thor & Loki as children

"The Master Plan of Mr. Hyde!" / "Tales of Asgard: The Storm Giants"
Mister Hyde, having disguised himself as Thor, is committing crimes. He then attempts to steal a nuclear submarine. When Thor tries to stop him, Jane Foster, interferes because she thinks that Don Blake is still a captive trapped by a bomb, to which only Mister Hyde knows the location. This allows Hyde to escape, and to make Odin feel that she is unworthy to wed Thor.
Release Date: 1963