I'm in 3rd/4th grade, and have two moments I can share, although I can't recall the exact chronology:
1. I am watching a Spanish soap opera. I remember the "galán" was a Paraguayan actor very popular in Argentina. He drinks a loaded drink (the evil antagonist would use the same tactics that our beloved rapist in the news right now) and fell asleep. I got an immediate erection, but I did not know what was happening to me.
2. I dream that I was traveling in one of those Mississippi River steam boats, and meet Batman and Robin (Adam West B&R). I am the villain and take them and try to drawn them in the orange juice container on top boat bar counter. I wake up with a full erection without understanding what was going on.