The term 'playing doctor' implies normal early childhood (i.e., pre-kindergarten/first grade) curiosity about bodoes and particularly the genital areas. It is alarming that an eleven-year-old would engage in this type of behavior with a five-year-old. Have you notified the parents of the older boy? If not, please do. The evry fact that a boy twice as old as your son engages in this type of behavior is coercive, simply because of the age difference. There do not have to be overt coercion or overt threats. Continue to st firm limits on this type of behavior with your son, and be vigilant. But do not regard the episode as normal childhood curiosity - it was not within the bounds of normal.
I have a very different take on this, because I don't think there's anything particularly alarming about what happened. Your son wasn't freaked out about it, and best of all felt comfortable talking to you about it. I think that's a sign you have a good, solid, healthy relationship with him, and that should be encouraged and cultivated. So, you might read "Sex and Sensibility: The Thinking Parent's Guide to Talking Sense about Sex", by Deborah Roffman, and "Harmful to Minors", by Judith Levine. I think they'll help guide you in dealing with this and other issues.
Yes, yes, to have good relation with Mommy and trust her telling her things is wonderful. The matter is the 11 year old boy that finds playing "doctor" with a 5 year old attractive and the playing becomes the exploration of genitals.
As noticed here, a 5 year-old has no inhibitions and problems to explore sexual parts because he does not understands exactly between right or wrong, or the significance of it. From there the reason why was so easy for him to tell mommy about it. For the 5 years old the play was as normal as playing with a ball and he would have continue playing in front of an adult or at his day-care or school.
Now an 11 y/o knows genitals are private parts and should not be uncovered in front of others or allow anyone to touch them. The 11 y/o would not play such a game in front of adults or at his school because he knows it is wrong. So, there is a BIG difference between his (5y/o) play with the 5 y/o girlfriend and his play with the 11 y/o.
You are very right MOM to be worried about the age differences and you would be correct to follow the recommendations of the first post about sharing the info with the parents of the 11 y/o and make clear you do not like such a game to happen again. And this is a good time to start introducing your child regarding his private parts and how he should keep them out of games. May be the 11 y/o intentions were nothing but explorations. I am not suggesting the older kit is a pervert. I am saying that your 5 y/o should not allow anyone to include his private parts in a game. He should learn now they should not be touched by anyone (except of course care givers responsable for his grooming)
Think of this. If this 11 y/o was able to explore and your baby went along, any one will be able to play with him with God know what intentions and he will take the game as normal.
I really rather to set very clear boundaries about this matter. About this matter, one is never too careful.
Your boy went alone and was not threatened because he does not know or understand the "other use" we adults give to our private parts. The 11 y/o however does know and I am sure he would not play like that with your son in front of you.
I definitely would take matter to the 11 y/o parents and get my son away from him.