I also have this problem with my U-10 boys. There is a drill which I use that they love. For younger kids we call it the Numbers Game, for older kids it is the Toughness Drill.
Set up a 20 x 20 yard square. Split the team in half and line them up opposite each other. Assign each kid on one team a number, say 1-8. On the other team do the same thing. You can match kids of equal ability, or make it lopsided to work some kids harder that could use a kick in the butt! Whatever you want, you can always change the numbers every so often.
To start simultaneously roll/toss/kick the ball, again whatever you want to do and call out a number "2!!". Send it far so they have to sprint 40 yards or close for a close quarters battle, vary the distance of your toss. A point is scored when one player passes it back to his teammates who control the ball cleanly. 2 points are given if you dribble in control over your own line. After a while call 2 or 3 numbers. PLay to 15 with losers doing sit-ups or whatever you choose.
If they don't move in this drill,, they may never go hard to the ball!