Congrats!!! Always a great day when you can save your own money by trading free gold.
Just a suggestion from a fellow dad, but you may want to take a look at the X-Terra Elite for your son. I just picked one up to augment my Sovereign Elite. I was initially looking at the Manticore, then NOX 900, then NOX 700. Then I also added the X-Terra Elite after I learned it was also Multi-IQ and waterproof. Money wasn't a concern, I just didn't want to overpay for features that I don't need or probably wouldn't use. When I compared the specs of the X-Terra Elite, NOX 700 & 900, I noticed that they're all almost identical, with the exception of a few features and different shafts (carbon fiber on the NOX 700 & 900).
I'm convinced that these detectors all use the same processor, pcb, electronics, etc. I think what Minelab probably did was to design the NOX 900 first. Then, by simply programming the firmware to limit or remove certain features, they have the NOX 700 & X-Terra Elite. I think the X-Terra Elite will detect as deep as the NOX 900 & 700 and watched several videos where that was shown to be the case.
But for a 10-year-old, I think he would appreciate the X-Terra Elite, as it would probably be easier for him to learn and use, less choices and features to deal with, but still basically getting the same detector (Multi-IQ, waterproof, wireless headphones, red back light screen, flashlight, etc) and more than likely just as deep as the NOX 900 & 700. I got my first detector when I was 10, back in the 1967 timeframe, we had to solder and put it together, lol. Then got a Whites Coinmaster IV about a year later. I wish I had an X-Terra Elite back then, lol.
BTW, I got the X-Terra Elite Expedition Pack, which cost $599.00, the detector by itself costs $479 but doesn't include wireless headphones. But if you go with the Expedition Pack, you'll also get a second coil (8"), ML85 Wireless Headphones, and Minelab's newest Pinpointer which sells separately for $149.00. I was planning on getting the pinpointer anyway, so getting the Expedition Pack saved me $149.00 on that, and getting the 8" coil and wireless headphones included saved me a total of close to $300.00 if I had to purchase everything separately. The X-Terra Elite Expedition Pack will give your son more and will save you hundreds of $. Just a thought from a fellow dad.