The whole paradox with God and the Soham girls is exactly the same issue mentioned in all the paradoxes and is fundamentally broken the second you consider Buddhas's point.
So, look at the simple paradox of me going back in time and killing my Grandad. We instantly cause the issue we are looking at with God and Omnipotence but within a framed reference that we (The human race) can understand. As Robbo pointed out (ditto Buddha) Omnipotence is our interpretation for a power without limit. Well, we can have no real conception of that.
Ok, so quick thought experiment. God created the universe. He was there on day one and God is there at the end. So for God, time has already occured (and don't forget, time is a human concept, rooted firmly in concepts that we understand). There is no past, present or future. Time has completely existed and his omnipotence means that there is no assumption.....ever. So, if this is the case, does God exist within time or without? If he exists within time, then he is constrained by his own paradox (He created time/the universe etc but is part of that very fabric, so how could he create?) If he is without time, then the paradox does not exist because all points within time have will already occured (I think thats a new tense "past,present, future participle"). So, he is omnipotent as all events are within his domain to do with as he see fits......no time framing reference required.
So, we view the events in our past, in our timeline. That means for us, there is a paradox, we remember those events, so if we change them, then we fall into the paradox. But God can't, because, if he is truly omnipotent and therefore existing outside of time itself, he created time. We just cannot think "outside the box" enough to envisage that in terms we can understand.
"God does not play dice with the universe" sort of sums it up. I am not saying this is the definitive answer but adds something to the debate.