Back in August, I bought my boy an 870 Express Youth Model in Synthetic, has the stock extensions with it to go from 12" to 13" LOP. From what I understand, I also get a standard length stock for it later on.
Basspro had them on sale, plus a Remington rebate and 10% off if you signed up for a BP card. So I got a pretty good deal at it, right at $200.00.
We have not shot it yet but it seems well made, solid. I like how easy it handles, I may shoot it this year.
It is still a little barrel heavy for my boy (he's pretty small framed for 9 years old), so he probably will wait until this coming summer to shoot.
I decided against a single barrel for all the reasons others have stated here & in the past: dont like hammer guns for kids (heck, when I was a kid I didnt like the hammer) and they kick like mules. I've got a cousin & a buddy who both let thier boys shoot a single barrel 20 as their first real gun to shoot, both of them are now "gun-shy" and not showing much interest in shooting shotguns anymore. Real bummer since both of these guys are big duck/small game hunters.
Single Barrels may be cheap but I dont see them as a value for kids.
BTW, Kent makes an all purpose 20 guage steel load that moves out at ~1450fps, around $9.99/box at BP.