It depends on what your child is like... we had DS go in at 3.5 for various reasons, and our psych said to be careful to say that he will do interesting activities with them, not games (play for DS always includes toys like trains)... so we told him that he is going to meet a few people that are very interested in meeting him and in doing activities with him.
We got there a little early so we could use the restroom before the testing, and let him get comfortable while we were in the waiting room. But DS tends to be by nature a calm observer and tends to love going to new places. And we did not want to emphasis the testing so we did not mention it until a few days ahead of time. We worried that if we mentioned it too much, he would get tense instead of being excited by something new.
They should know how to handle all types of children, and be able to work with most children to keep it fun and smooth. DS loved the whole process, and our psych mentioned that he detected DS had initially sandbagged one sub test, and was able to coax him to cooperate fully later in the session to get a more accurate assessment.
good luck!
