I believe your gamble is doomed from the start.
Last I heard, Steam has the very same policy, that no characters under the age of 18 may be present under any circumstance in R+ games. I know that from another game, Goodbye Eternity which had to modify a non-sexualised under 18 side character into a +18 one to put the game on Steam. I also know that other games, to solve this issue, either give children in their games "miraculous growth" or just mention them without adding any depictions (at least not officially).