They stuck at this with minimal interference from me for what must have been approaching two hours. My job seemed to be merely fetching more nails and screws. They were so into it that we ran out of screws and I had to resort to giving them a box of white plastic coated hooks that had screw ends to keep going with.
As usual, the combination of being allowed to play with two things that they usually aren't (rocks and nails) meant that this was a very popular activity. I think they inadvertently got a lot of skills from this though and it was free because we had all the stuff lurking in the garage. Yay free! I'm just going to pile the boxes in the garage and if they ask to do it again tomorrow or next week then I'll turn the boxes upside down and give them some smaller screws and nails and see how they do with something a little more fiddly. Mwah ha ha!