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.