Dean tugs his jacket on, pats down the pockets (it's an adult's mannerism, one he's picked up from Dad, and it doesn't look quite right) – gloves, money, door key... He glances back over his shoulder at where Dad's left the shotgun, bites his lip as he considers... no, that's just stupid. It's too big to carry without someone noticing, too substantial to be taken for a toy, and the last thing he needs is for Dad to get back and find they've been arrested or something. Still, he feels safer knowing it's there and not taking it only helps drive home that they really shouldn't be doing this.
This is so not his best idea ever.
And it's way too late to back out now, right? Sam would never, ever forgive him, and one thing he's learned is that for all they tell you little kids will bounce back from things his brother can hold a grudge like nothing ordinary – true, he'll be back to clinging and grinning like a maniac and giving you the toy out of the Lucky Charms within an hour or two, but days later it'll surface as evidence backing up why you totally suck and should go away and die or something. So he gives Sam a thumbs up - “Yeah, ready.” - and catches up with him at the door, ducks in front of him to block Sam from just dashing outside now they're ready to leave, tries the deadly serious face again. “So when Dad asks what we did for New Year...”
It feels bad, encouraging Sam to lie – it's not so very long ago he was at that age where everything's black and white, and the fact he's now old enough to understand that sometimes a white lie is okay stings a little in a way Dean doesn't really understand yet because he's not really old enough himself to grasp the idea of time going too fast – worse still that the person he wants him to lie to is Dad, but it doesn't hurt anyone, right? All he's trying to do is give Sam a New Year which doesn't totally blow to make up for a Christmas which really did.