Sorry, I didn’t see this until now. 
There’s the two methods stated above^ the first is known to be the easiest, especially if you want the MC to somewhat look how you as the author want them (like deciding their hair and face shape or whatever).
I’ve heard that this also changes the character’s outfit to match the original character, so if you try that, it’s safest to change the younger version afterwards into their respective outfit. You will also have to be weary of the lip color because if the original character is wearing lipstick/reader chose lipstick, then the younger version will also be wearing lipstick.
Some authors have @YOUNGCHARACTER changes mouth into Full Round Pouty etc etc. underneath every single feature in the first CC menu where they’re customizing their own character, so the young version changes as well as the original character, but this method is tiresome. I’m certain someone would have a template though. If you don’t get what I mean:
@CHARACTER changes mouth into Full Round Pouty
@YOUNGCHARACTER changes mouth into Full Round Pouty