vertical, child
豎 centers on the idea of something standing upright, like a vertical post. The child meaning is a separate, unrelated sense that does not connect to verticality.
豎 combines 臤 (a phonetic component) with 豆 (a vessel or bean). The vertical meaning likely comes from the idea of something standing upright like a vessel, while the child meaning may be a separate loan usage. The exact historical development is uncertain.
The top 臤 looks like a hand (又) holding something firm (臣), and the bottom 豆 is a standing vessel. Imagine a hand steadying a tall bean-shaped vase so it stays vertical.
For ジュ, picture a child (one meaning of 豎) wearing a jewel (ジュ) around their neck, standing upright like a little vertical post. Jewel -> ジュ.