losing baby teeth, child
齔 centers on the stage when a child loses baby teeth, so it refers both to that process and to a young child at that age.
齔 combines 齒 (teeth) with 匕, which likely contributes a phonetic element. The character specifically denotes the shedding of milk teeth in childhood.
The left side is a tooth (齒), and the right side looks like a small spoon (匕). Imagine a child using a tiny spoon to wiggle out a loose baby tooth.
For シン, picture a child losing a tooth and showing the gap with a big grin: grin -> シン, and the child is at the tooth-losing age.