And yes not even Superman can lift Mjlonir with pure strength alone. Just like the Hulk could not in the Avengers.
This I don't buy. First of all, depending on what you want to go with Superman is strong enough to lift the planet
and then some. He should be on a strength alone be able to pick up Mjolnir. But, fine, we'll simply go with the magical enchantment on Mjolnir means it's not a matter of strength to pick it up, it's worthiness. And if a child were "worthy" they could pick it up.
And could anyone really be more "worthy" than Superman? He's as good, unselfish and as kind-hearted as you can get and not as prone to bouts of stupidity-induced egotism like Thor is. Superman should be able to lift Mjolnir but we'll roll with what Thor says there, I suppose, that on some level Odin/Mjolnir itself decides when, why and who is worthy and even then maybe let them get to lift it.
Yeah the current era Superman (Post Crisis on Infinite Earth's back from 1986 up to New 52 and Man Of Steel) doesn't have planet moving strength like his Silver Age incarnation. The Silver Age Superman and Superboy were capable of moving through time under their own power as well through use of super speed. The present era Superman tops out at 700 mph or the speed of sound. Different eras of the character account for the discrepancies of their abilities.
In the JLA vs Avengers comic where Superman and Thor fight it's post-Crisis Superman.
Apparently Thor can pass the Mjolnir to someone else if he chooses.