Heat a serving size to a point where it is a bit hotter than your child normally likes their soup (to allow for some cooling during the morning).
Pour the soup into an unbreakable vacuum flask of the same size so that it is full (this reduces the amount of air and the soup will stay warm longer
Screw the top on tightly so it won’t leak.
For other anti-leakage measures include:
Place the flask of soup in a plastic bag and close with a wire tie.
Wrap the plastic bag in a handtowel (also provides more insulation so stays warm longer and can be used to mop up in the event of spillage)
Put it into your child’s bag so it stands upright, at least when it leaves home!