Can hard physical work as a kid make you strong for the rest of your life even if you become sedentary?

no... ur muscles will eventually degrade. bones il epmporarily become tougher but ofer time theyw ill wear back to normal also if u work with heavy weight resistance u can hurt urself or becoem deforemed. as the bones are still growing an ul alter their shape . bones are metal believe it or not. actually a calcium rust of sorts

so they wil be malleable to a degree

I don't think so-if muscles aren't exercised consistently they will loose their strength.