My #1 Tip for traveling with children is... ENTERTAINMENT!
Tip #2 Snacks! Make sure you have enough snacks for the length of your trip. If you are driving, limit drinks, but have them on hand!
Tip #3 Utilize Rest Stops! Kids have bladders a lot smaller than our own and may need to use the rest room more than we do. Make sure you are taking enough rest breaks if you are traveling by car. If you have younger children that may still be able to use "pull ups", I personally would use those for extra assurance in case of an accident!
Make sure your child has enough items to entertain their time whether traveling by car, plane, train or bus!