Layups require a lot of skills in a short period of time. Kids tend to release the ball at the speed they are running, and off the basket it goes. A problem is that they release the shot with their hand facing the basket (like a shot), normally you would turn your hand around under the ball.
a) To start, have kids jog around the gym, every few steps, jump off a foot, lift the other arm, land on the same foot.
See Footwork - Ganon Baker layup form.
b) Wall layups - have each player face a wall, step forward, release the ball (opposite hand), catch, step back and repeat.
See Form layups.
c) Partner layups - two players at a basket, each with a ball, the first player takes 2-3 dribbles for a layup on the right side then his partner goes, shoot five each, repeat on the left side (strong-hand layups are OK, go easy on off-hand layups for kids).