I’ve been drawing these blue “increment arcs” (what I’ve been calling them) on a separate layer, where plan out the inbetweens pose-to-pose, starting from the midpoint, then halfway to the midpoint etc. You could also draw them for something moving in a straight line, or any shaped line, but so far I’ve only had animate arcs. It’s kind of like a timing chart in context. Then I draw the lines for the top of the head straight ahead by joining up those points on the arcs, which results in this:

The perspective for the top of the boy’s head changes through this movement, but I don’t have to figure that out because it happens naturally by following the lines on my increment arc. Then I draw the rest of the body by eye. With this method, I’m hoping the movement turns out smooth and consistent without looking too mechanical. Here’s the same thing for the dad: