It may not necessarily be socks. While you'll have outfits packed for your kids for every day you'll be gone, there is inevitably going to be some minor garment that falls under the radar: socks, underwear, undershirts. It's okay, I'm sure no one at Disneyworld will be able to tell that your kids are going commando, except for how your kids will proudly announce it every seven seconds.
2. An extra package of Wet Wipes
It doesn't matter how many wipes you think are left in the package you start out with, you will find yourself pulling the last one out of the container within an hour of leaving your house.
3. One variety of diapers
Regular diapers, overnight diapers, swim diapers - at least one of these will still be uselessly tucked away in the changing table just when you need them. Don't feel bad, unless you actually forget to put a diaper on your child before cramming him into his car seat.
4. The iPad charger
It will die with ten minutes of How to Train Your Dragon left and the screams of Vikings in battle will be replaced by the screams of furious four-year-olds.
5. Your kid's lovey
After waking up, shoveling breakfast in everyone's face, and hitting the road ASAP in the morning, certain things get left behind. Certain things that will ruin your life when you try to put your kid to bed the first night away.
Somehow, taking your children on the road opens an all-consuming black hole inside their stomachs (and yours). It is not physically possible to pack enough food into a backpack, or even a mini-van, to last for an entire journey.
7. The nail clippers
You might have packed them, but not anywhere you'll be able to find them when your kid rips a vicious hangnail in the middle of I-80.
8. The dog
Odds are good you'll notice how quiet the car is just as you're crossing into the next state. You knew you shouldn't have put her outside for one last bathroom trip before leaving home, but you thought you'd be saving yourself a pit stop on the road. It's okay, I'm sure she'll enjoy spending her vacation in the backyard.
9. Enough layers
Someone is going to wind up sitting directly under the airplane's blowers, or right in front of the car's AC vents, and their comfort is the necessary sacrifice to keep everyone else's face from melting off.
Not for the kids - you'll wish you had packed an extra one for you, to comfort yourself in the middle of another forgotten-item panic attack.