Always great to see Dads introducing their kids to shooting sports....when you get to the point where you're adding a dog to the mix, keep us in mind,
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Over the last 15 years, I have trained over 500 children under 5 years old and 1,000 children over 5 to work their dogs, many of whom were adding bird hunting to their hobbies. For the value, I recommend an economical, lightweight, and rugged shotgun. Yildiz, a lightweight over/under Turkish-made shotgun that will rival the performance of guns 4X the price. You can pick one up at Academy for less than $500.00. They do have models in 410 and 28 gauge. (P.S. I'm not getting paid for the endorsement, although I should be given the number of people I have sent to Academy in the last 15+ years)
Depending on what load you are shooting and the size of your 9-year-old, I would highly consider a 20-gauge. You will find that the cost and availability of ammo in 20 gauge is much better, and a 20 guage can easily compete with the big boys shooting 12s, depending on your shot proficiency.
On the lighter skeet or bird loads, the recoil is tolerable for a 9-year-old, depending on their size. When you get into the heavy loads for duck and goose, an over/under will have more recoil than an automatic that uses the discharged gas to help eject and load the next shot, however the weight of an automatic will be significantly more especially when lugging it throw muddy fields or marsh. You will give up the third shot option; but after guiding for many years, statistically for most shooter, that third shot is extra money for the sporting good store.
Here's a suggestion that will make the biggest difference. Spend the dollars for at least one session with a shooting coach; or at least reach out to the A&M Shooting team and find a student that would be willing to offer a session for a case of shells!
The one drill a shooting coach offered years ago was to drop a flashlight down your barrel and just practice the mount with both eyes open until your son or daughter has the muscle memory to hit any spot on the wall with the light beam mounting quickly while standing, or sitting, simulating a hunting environment. KEEP BOTH EYES OPEN!
This drill will save you hundreds in ammo, and your kid won't be apologizing for letting a curse word slip after an easy missed shot!
Then spend the money for a sporting clays round with a coach. This is worth several cases of shells wasted by yourself with a thrower.
Have fun... There is nothing like watching a kid get hooked and proficient at shooting sports...and unlike golf, you are coming home with something to eat.
When you are ready to add a retriever to the mix, that's when the real fun begins. There is nothing like watching a dog work the field or ponds for a downed bird.
http://www.HuntersCreekRetrievers.com 
