Our DS just finished his 5th (and last) pre-k/k visit, and all indications are that each went well. We used the very low key approach, and it worked for him/us.
We told him that we needed his help picking his next school. And each visit would give him a chance to have fun with some other kids, possibly make new friends, and meet new teachers at each school we're considering for him. After each session, we then asked him whether he had fun, and did he like the other kids, the teachers, and the classroom.
Anonymous wrote: My DD does not always separate well . . .
We had the opposite separation problem; "I want to stay longer" was his usually response. After the first one, we had to work with him a little so that he wasn't too disappointed when each play date was over!