I'm not sure if you are driving or flying - it makes a big difference with what and how much you can pack.
If you are flying, travel as lightly as you can. Pack with the idea that you can (hopefully) do laundry at your destination. Don't worry about packing loads of diapers, wipes, food, etc. because you can always hit a store once you arrive. Since the airlines are charging for every bag you check, it is expensive to bring tons of stuff. And if you are bringing car seats and a stroller, those can be checked for free. As far as what to bring on the plane, again the idea is to go lightly. Like others have suggested, bring a change of clothes for the kids and for yourself. Have enough diapers for the flight, and then bring several extra just in case. Bring enough snacks to keep your 4-year old happy (I always bring cut up fruit, string cheese, goldfish crackers, and a Uncrustable sandwich, which is a BIG deal because my son only gets those when we are flying) and an extra bottle for your 6-month old. You can take liquid through security if you are traveling with an infant, but you'll have to pull it out of your bag and you may have to go through an extra check. Bring a few quiet toys to play with, a few books to read, your kids' lovies, and if you have it, a DVD player with a favorite movie for an emergency. You'll be surprised how easily kids fall asleep on airplanes. Nine times out of ten, my son conks out within 15 minutes from takeoff. It may seem like a lot, but I can get all of this into a regular size backpack. And have your four-year old help by carrying some of his/her own toys in a rolling backpack or small suitcase. Then s/he will feel like a big kid by helping Mommy!
If you are traveling by car, then you have much more flexibility with what you pack. We have always also tried to pack lightly when we drive so that it is not a production to unpack when we arrive at our destination. But we are able to bring more comforts into the car, pack a few more sets of clothes, and not have to worry about paying the high baggage fees!
Try not to worry about it too much. It is fun traveling with kids - good luck!