I'm amazed at most of the answers here.
These are little kids - under 10! They looked, they found their gifts - regardless of what I'd do, this is normal kids behaviour.
In Ireland, these gifts would be Santa presents - we don't have the stocking tradition as such (well we do, but they really just have sweets in them, not gifts), so the main present is usually Santa's. Some families buy from parents, some don't - these will be smaller presents.
So if this was the case, they'd have to be replaced I think, or some very good story concocted - not entirely sure what, and it depends on how young they are!
If they are gifts from mum & dad, that's different of course. I do agree they will still get these gifts; I might find some surprise to add to them - or again it depends, if they are having stocking gifts from Santa that are a suprise, that takes care of that.
But overall, they didn't do anything so terrible, and while they were looking, most likely didn't really think through what they were doing.