And PP - I would always recommend focus on the loading position forward rather than focus on the starting position - so starting from 90-90 is a very good way to isolate the specifics of the approach and acceleration into contact. I have taught countless players to work forward from this position with no issues around stiffness or ruining a motion.
I think this is what I'm doing with my kid. Although he is quite advanced in his game for his age, he's now starting his serve at the trophy pose (then coil a bit, and toss and tilt back at the same time). I'm basically following Rick Macci's advice when he works with younger kids (isolate certain aspects first).