Can someone cite for me the page number in the rule book which says that if a mini is prone and then engaged they can take their move action to stand up. The only prone -> standing rules i can find are

1. Stand up as a move action this cost half your move allotment
2. If you win a duel you automatically stand up

I find nothing which says you may take a move action to stand up after you have been charged. In fact, i see on page 22 of the small hobbit rule book that once a model has been charged he cant move, which means to me he cant stand up until he either wins the duel, or is disengaged for some reason.

Am I wrong?

No. You are right. A prone model that has been charged cannot move or stand up (which counts as a move). This is why it is a good idea to call a heroic move the turn after your army has been nature's wrathed, say.

Hee hee. I just did that today. My woodelves were surrounded so i rushed thranduil in there and nature's wrathed all the ferals and berserkers surrounding my warband. It was great! Sure it knocked all my guys over as well, but it was great knocking over some 12+ minis in a single go.

It was actually that scenario which spurred the question. After the action, the following turn, I had initiative so my guys all stood up and everyone asked, what about the uruks? We stopped the game to try and find an answer for some 20ish minutes. In the end we interpreted the rules as above and pressed on with the intent to question later.

Why were your guys knocked down? Natures wrath only knocks down enemy models.

Not according to the spell description. The spell description says it knocks over everyone within 6 inches of the caster

I'm pretty sure it only knocks prone enemy models.

You're right, my mistake. It only effects enemy models. Even better!