I also would just leave it as "great job!" and not mention the meltdown or that he should focus, and then just leave it alone for now. He has been there once now, and as long as he did not leave telling you he hated it, I would avoid any more references until a day or so before with just a "say, tomorrow we are going back to do it again, remember how fun it was?" then move on to another topic.
trust me, good testers should be able to pick up what is going on. I read an old report from when I was tested at 11 or so, and she clearly wrote that she felt one sub test score was lower due specifically to my attempts to rush through it because it held no interest for me at all and other similar comments that indicated she had a good idea on my weaknesses and how it affected a few subtests that were lower than what would have been expected based on my other results. And with 3-4 year olds, I am sure they have seen all levels of cooperation...
