I played doctor with about three kids my age -- two boys and one girl. One boy (a friend) and I did it several times between ages 8 and 10 or 11. I know at least one girl a few years older than I was did it to me ... might've been a babysitter?
As for my friend ... I remember I had some pretty specific ideas on how it should go ... based on my own experience at the doctor. He had some too I guess.
The examination would begin fully clothed. The "doctor" would instruct the "patient" to remove his clothes bit by bit until he was in only white briefs (we both wore white briefs).The doctor would then slip them down slowly and examine the penis and testicles closely. My friend always wanted this part of the exam to be done while the patient was hard. I wanted us to be flaccid ... because it was more realistic I think.
Examination of the anus was included when he was the doctor -- I was made to roll over and he would spread my cheeks apart. I started doing this to him too but only because he liked it. I never really did.
It's interesting how these same preferences still "stick" as an adult. The idea of sex during an exam turns me off, and I've never had much of a fixation on the rear end.
We stopped doing this before we hit puberty. In some ways I wish we would've had a session or two afterward but we probably were too chicken. We certainly saw each other in the school showers after adolescence and he developed a larger penis than I had. After seeing him naked during the day, I'd fantasize about him at night and imagine the doctor examining his bare penis and balls.