Background and caveat. I am that uncle the parents fear. I let my nieces and nephews do about anything they want - until the wife-unit shuts it down...
1. Note any disposition for science or engineering - it was obvious very early (pre-school) in a particular niece and nephew. One other did not develop said disposition until well into high school.
2. Provide an environment. A nephew used to play with my o-scope and function generator during his early grammar school years and played with other stuff on my bench. He later watched me model stuff using MathCad in the 90s. And he recently got (yet) another degree in computational physics.
3. Be logical. Allow curiosity. Allow and never disparage supposedly 'self-evident' questions.
4. When a STEM interest is noted, be ready for the "what is the difference between science and engineering?" question. Do not mistake a STEM interest for a desire to pursue a STEM career. The others nieces and nephews are, or will become, ranchers, business people, and artists; and the artists were probably the most intelligent of the lot.
5. Exposure to, and enjoyment of, an activity are not the same thing and will not necessarily result in similar outcomes.