It’s obvious: Before doing practice you should always warm-up properly. When coaching my U14 team, they usually think of warming-up as being an unnecessary chore, therefore they seldom do it on their own. Instead they wait until I tell them to.
Children want to learn why they have to do something the coaches tells them, so this is a great chance to combine theory and practical experience. Make use of the childrens need to move and prepare the warm-up with small games and competitions.