My son has just turned four, this all started when he was three years old,
It started just before Christmas I found my partners son rubbing his bottom on my sons belly, I thought at the time it was innocent curiosity, that my partners son is six and was acting out something that he had seen (neither of them have ever seen me and my partner and we do not watch anything inappropriate in front of them, but we cannot speak for sons father when he visits him and my partners sons mother who he lives with) it was extremely embarrassing for us both and we acted immediately. We took them both aside and asked them what it was they were doing, his son became very distraught blaming my son saying he asked him to do it and it was his game, with this we calmly asked my son trying to keep things light explaining that he wasn't in trouble and that mummy was just trying to understand what they were doing, he explained everything as calm as you like that this was his game, it was called willies and bums and that he learnt it at nursery with two other children, he then went into great detail of how the game was played and that it involved putting their fingers into each others bums, I asked how they were able to play this game at nursery and he explained that they did it in the home corner under the pillows, I explained to him that this wasn't a game and that it was inappropriate and that he shouldn't do this again, that he or the other children could get hurt, I rang the school straight away and told them the situation and said that I would be in to talk to them about it the next day. I went to the school and all the correct actions were put into place, the parents of the other children were told, the home corners in both pre school rooms were taken away and made into an open space play area and all of the staff were asked to right statements and keep a record of each step the children made throughout the day I.E who they played with, what they played, what was said etc.
My son came home and told me that one of the children that was 1st involved in this asked him to play it again and that he said no and told the teacher, with this I asked if my child could be moved into a different room to try and solve the problem,
About a week after my son was moved the school pulled me into the office to explain that an incident had happened again, they explained how my son was caught in the toilet with a boy that was not involved from the start, that he had his hand on his bottom when the teacher walked in and stopped the situation, that she explained to him that this was inappropriate behaviour and not excepted at nursery, we then had another chat with him at home that this behaviour could not happen again as he is to little to understand and that it is upsetting to others,
A month after that my partner caught our sons together again in their bedroom, we sat them both down with each other and asked them to fully explain why they were still doing this after they have both been told not to(we are obviously very upset and mortified beyond belief now and are at a loss of what to do)they again went into great detail of how the six year old placed his finger up the four years old bottom then it was his go for the four year old to do it to him that they also poked each others willies and then went onto putting fingers up each others bums again when my partner caught them, my partner was to overwhelmed at this point and left room, I went on to explain to them both of how serious this was and that it needed to stop immediately, that we would not except this behaviour anymore as it is inappropriate, offending and extremely harmful.
Two months passed and we started to feel better about the situation until last week when I was again called into school, my son had again been caught touching another little boy again a completely different child to any of the other times but this time it was in the middle on the room, they were caught and stopped straight away and the teacher spoke to my son again about his behavior, I decided not to do anything that night at home as the teacher dealt with it at the moment of it happening and I didn't want to bring it back to his memory and also because I have no idea what to do or say now,
I'm am terrified for my son and at a complete loss on what to do or where to go from here, can anyone help me I have heard and read so many things for this happening to children aged 6-8 but nothing for a child so young even the nursery have never heard or had to deal with this situation before and they are also unsure on what to do next ?

What would I do? I'd take my child to a professional therapist, first to determine if he had been molested, second to determine why the behavior continues. I'm sure you've been told the worst thing you can do is over react (and I agree, how can you not) and make innocent play a great big deal which then becomes secretive.

You should not be "extremely embarrassed." It's not like you are teaching him this behavior. You should, however, be concerned and you should be discussing this with his father. Unless you are with your son 24/7 you don't know what he's watching on TV.

I don't know that he's too little to understand (including at age 3) that there is inappropriate touching and inappropriate BEING touched. I have a 3-year old Grandson. He understands. I don't know what he would do BUT he does understand what is inappropriate.

I have a similar situation with my 4 yr old and was looking through the web for help on how to handle that and came across your post...

Its an old post, Has your son grown out of this phase, what did you do? I am feeling totally clueless, I don't know what to do or how it started with him...

I also came across this publication : http://www.hss.state.ak.us/ocs/Publications/pdf/sexualdevelop-children.pdf

Here in a table it says its normal behaviour for kids at this age, a little consolation but I am still jumpy.

Hi I would love to say yes but unfortunately no his not grown out of it, me and my partner have no idea what to do, we had thought it had stopped a while ago but the weekend just gone we caught them doing it again, we sat them down for 3 hours and told them what would happen if they did it again and we didn't sugar coat it at all in fact we went over board a little bit but we just don't know what to do for the best now. The chat might have worked but only time will tell. Im sorry you are going through the same thing and I wish I could give you good news