I have three suggestions:
1. Bounty Hunter Tracker IV: This machine has great discrimination, is lightweight, and does an all around good job. Depth is not spectacular so you won't get over 6" on a quarter with it. But depth is reasonable for the price. Hobby Lobby often has a 40% off coupon which brings the price down to around $60-$70. At that price it simply can't be beat as a starter detector. Because of the depth he will want to upgrade sooner down the road. But if he loses interest you aren't out much.
2. Tesoro Compadre - Good machine for decent price. Pretty deep but very little in the way of controls (one knob). Around $160. Personally, I'd pay a little more for a better machine but this one is good if you are really price sensitive.
3. Tesoro Silver uMax - Great machine but more expensive than the other two. This machine is really one of the best values you can get on a beep and dig machine. Around $250 but worth every penny. This thing is deep and has a great discrimination control. Just enough controls to make it powerful yet easy to learn. The problem with this one is that you may end up wanting it for yourself. Countless people have thrown away their "better detectors" because the Silver uMax is just so easy to use and does such a good job.
I would suggest sticking with a beep and dig machine. The lower cost machines with target ID like the Ace 150/250 really don't do that good of a job with target identification. This will lead to missed targets and frustration. They also take a little more effort to learn and a 9 year old might not be able to really grasp the nuances. That's my two cents. Good luck.