Normal i dont know lol . BUT i have heard of it alot . When my son was younger he come running in the house saying in the shed outside 2 of his friends was pulling each others pants down this freaked my son out , i have heard other people have the same things with young ones . Now if it keeps happening then thats a different story.
No, that would not be expected, and likely occurred because one of the boys had seen something (could have been a picture vs real life situation) or heard something to this effect. It defibnitely is worth following up via questions to the boys.