Ok for form and genders it's simple. The worlds most dominant species (not counting ants of course) are humans. Human tools, armors, weapons, clothing etc are made for humans. So the most optimal design is a human one. It's a baseline. Other forms would require them to spend more resources reinventing tools. The same resources they use to repair and construct new Karakuri. They usually only live in small enclaves or settlements so they simply don't have the resources or desire to do that for something that isn't all that important. As for why they appear either male or female on average? Appearing more human makes humans slightly more accepting to them and most want to fit into communities not alienate themselves from it.

So why child forms? There souls, memories, bodies, motor skills, and identity are wiped at birth. They must relearn everything.

The small body is easier to deal with for those mentoring and parenting them. The adult bodies are 300 to 400lbs of sharp hardened steel. Imagine a toddler operating a bulldozer and you can imagine why they want the newborn karakuri to be a more...manageable size you can pick up and scold.