Put the remaining food into the bin. **** This is correct ***
Be careful not to hurt your neck. **** This is correct ***
(told to the children who are hanging downward from a bar)
Watch out where you're going! ****This is correct ****
(told to a child who is running and nearly bumps into another child)
**** So, all three of your sentences were correct. But some of your comments in parenthesis needed to be changed.
I hope this helps!