This would really bother me too
The NSPCC has some good resources for teaching children about what areas of their bodies should be kept private for children his age.
Unfortunately children do overhear things, especially from other children who have older siblings or older children on the playground. It can be hard to pinpoint where these things come from... But it is important to find out if it comes from other children or an adult.
If you are concerned that there is no possibility this is coming from children talking and just finding what they know to be rude words funny, you should report it. Especially as it happened in your care if you genuinely believe the parents aren't handling it appropriately
However this may damage your relationship with the parents and you must
You also have to be very careful questioning children, any leading questions would not be helpful to social services or any safeguarding organisation as leading questions are know to put ideas into a child's head making the information void
So you would have to wait until something was said again and stick to where, who and when questions
Eg, where did you hear that? Who said it to you? When did that happen
I hope you are ok, try not to worry too much, I've heard some things in my time at school and they have all been traced back to other children so far