well she was about 8 years old, so she was a young kid - and kids when they are young go through the motions of procreating, usually alone but sometimes it becomes something that is shared with others they are playing with. This sounds to me, is what happened. I guess it's a child's first lesson to
be told by an older child to do something, and one kid would do it, and another would question it, maybe based on a "talk" given to them about possible situations where they need to take control and just say "no". You've said that she "tried" to shower and change etc with you, so that suggests that you stopped her from getting too close to you, and at 5-6 missed thwarting her advances and got coerced into something that was out of your control, and because it was sexual in nature, and because you had your control taken away from you, i think it would help you to find closure by talking to a therapist about it. As for telling anyone, do you mean your parents, or hers, or the child herself. Who would you tell ? This is a very personal thing, and if you have a friend or partner that are there to support you unconditionally, and you feel like sharing it, of course, get support from your loved ones. Talking to a therapist will help you to deal with an event that happened to you and maybe scarred you by having your control taken from you. Even the fact that you had to stop her from "trying" to get close in the shower etc. can be internally traumatic, so i think you deserve to look after yourself and have a chat with a professional about it. Maybe it will go a long way in you being able to truly put this behind you (better than by not going) . Nothing ventured, nothing gained after all. You're in my thoughts and prayers. Thank God for moms that check on kids.
And thank you for sharing this event, and having other mothers forewarned that kids should not be left on their own at that age, without being checked upon and without having a "talk" that would help kids retain their power, especially with other kids that a bit older.