I just had my 5 y.o. DS tested too (SB-V and WIAT). I told him basically nothing, mostly because I didn't know what the tests would involve anyway. I told him in the morning that we'd be going out in the afternoon, then when we got in the car I told him where (to see Dr. X who isn't the kind of doctor who gives shots), and that she would be asking him questions, and that it would be fun, and that I would be nearby but not in the same room. And I told him to answer her questions for real and not with made-up replies (he likes to be silly), and to do what she said. -- which was unnecessary, because as it turned out he liked the tester a lot and was going to do what she asked anyway. So, essentially no advance warning, and he took the whole thing in stride and did very well on the tests. And he is not the sort of kid who likes new experiences or being away from me in a strange location.
edited to add: I should mention that he has overheard me talking about the school hunt and at one time seemed to be getting anxious about it, so since then I have purposefully avoided that subject around him. That's why I didn't tell him anything in advance or that this was to help choose a school: I didn't want him to brood over it.
