I'm not surprised to see Dean being pretty good with kids.
No, we've seen him interacting a few times with young boys and he's always been great with them.
And I'm not taking points off for yelling at Ben when Ben got into the guns. There was a whole bit showing that Dean made a solid effort to keep the Impala guns locked away. (I'd be less forgiving if Ben had been messing with the gun under the bed.) Finding a kid within seconds of possibly blowing his brains out can justify some yelling.
I think that was completely justifiable yelling. But I also think Dean's deluding himself if he thinks Ben doesn't know about the gun under the bed.
Still, Sam's surprise at Dean's dad skills was a little odd. Didn't we have several flashbacks where Dean was left alone taking care of Sam?
Was Sam mostly surprised at his adeptness at baby-wrangling? Because we've never seen Dean with a baby before, and it's probable Sam never has either. And Dean may have helped take care of Sam when he was a baby, but Sam wouldn't have any memories of Dean when Dean was 4 or 5. And as low an opinion as I have of John's parenting skills (I know I'm in the minority here) I sincerely hope he never left a 5 or 6 year-old alone in charge of a baby while he took off on a hunt. In the SPN backstory in my imagination the boys were a bit older before Dean was given full responsibility for looking after Sam on a practical day to day basis.