Without a doubt, kneeling and throw over hand with as little arc as you can that still allows for the kids to get their bat around.
It's painful to watch adults throw overhand standin up to young kids. If they stand too far away they can't throw strikes and most of the kids have timing issues.
Stand too close and the pitch comes from such an incline that most pitches are too high and the kids need to uppercut to match the plane of the ball.
Machine pitch for kids even older is preferable to watching kids not throw stikes. It's not fun for the hitters getting hit and it's certainly not fun for the fielders.
Who cares about the outcome at this age. Emphasis should be on having the kids hit the ball so the fielders can spend more time playing and less time watching.