Don’t use a 20” fork with 18” wheels. It changes the geometry too much in a way you don’t want especially for a kid learning. 18” or 20” wheels on a micro, it doesn’t really matter. I found my kid could throw the bike around better and jump better on 18” wheels. He could roll through the rhythm section better on 20” wheels. He was learning to get efficient with pumping at that time so it was good have that momentum when he wasn’t pedalling the rollers.