The children will be learning that their body belongs to them and they should talk to a grown up they trust if they feel worried about anything. The aim is not to worry children, it is of course age appropriate and it's being delivered by an experienced NHSPCC member. The format will be, the children come in as usually for registration and then during circle timethe NSPCC leader will show the children a video and give them a very short talk. After that they will be doing a "pants" themed activity. Such as decorating pants or pin the pants on the dinosaur. The children will have a lot of fun doing this activity.
Please do have a look at the material on the NSPCC website. You will see the video there too https://www.nspcc.org.uk/preventing-abuse/keeping-children-safe/underwear-rule/