If you get the P6, your definitely going to want to change the mainspring. The DA triggers are pretty stiff. The P225 mainspring brings it back down to what you'd expect from most other SIG's. Check out top Gun Supply if you get one, they have all the parts you need and the Armorers CD to help you tear it down and reassemble it.
While the P6's grip is somewhat thin, its really not a whole lot different than the grip of a P226 or P229. I have all three and see little difference in them. I also have a P239, and as 1BLINDREF said, its grip is slightly smaller, and especially thinner than the other models.
If your willing to spend a little more, I would suggest finding a local SIG dealer if possible and let your son try all of them in his hand and let him choose. After all, its going to be his gun. There are deals on CPO's and used SIG's out there, but I doubt you'll find one as cheap as the P6's are going for now. Other than my P230's, the best I've been able to do has been in the $450 range for a used SIG above .380 in caliber.
I have a P6, and was wondering about converting it to DAO; anyone familiar with Sigs care to enlighten me on the possibility?
I would imagine if there are P225 DAO parts available, it would be fairly easy to switch. I have a P245 that was converted to DAO that I just recently restored to DA/SA. Just a matter of swapping the hammer and trigger and replacing the decocker in my case. In yours, you'd be eliminating it.
Edit to add... they do show a DAO hammer here, but nothing about the trigger. Not sure if the one in the gun will work with the DAO hammer or not. Maybe try over on the SIG forum for an answer there if you dont get one here, or maybe just call SIG and see what they say.
http://www.sigarms.com/Products/documents/SIGSAUERP225PartsList7-21-2006.pdf
This design is a good one if the decocker is used correctly..... I believed the gun was pretty safe, guess not.
As compared to what other loaded pistol in the hands of an unattended 3 year old?