Research what the trip is going to cost. Search the Internet for prices of hotels, prices of attractions and what either airfare or the cost of gasoline is expected to be when you want to embark on your family trip. Estimate what the cost of dining is going to be on your trip, and you'll have a reasonable figure to begin budgeting.
Determine a maximum amount you are going to spend on your family trip. Barring a minor miracle, your trip will cost more than the estimate, so consider the other entertainment expenses and the miscellaneous expenses you will be paying for, and base your maximum amount on that. Don't underestimate these expenses, or the likelihood you'll exceed that maximum amount is almost guaranteed.
Give your children a budget while on the family trip. Children tend to want their parents to buy things as they're going places, and family vacations are no different. Give them a certain amount of money for the duration of the trip, and help them spend it responsibly to increase their financial maturity.
Share the budget with your family, and explain the reasons behind your budget. This will be a good learning opportunity for your children to see how expensive things are, and give them a sense of the importance of budgeting money. It will also help you be more secure in your planning of the budget, as you will have to defend your reasons for allocating money in the budget.
Prepare yourself financially for emergencies, because problems can and will arise on any vacation--and some are a lot worse than others. Set aside a decent amount of money in case a hospital visit is needed, or if your automobile breaks down and requires a tow truck and a few days in a mechanic's garage. For added insurance, have a credit card in reserve that has no charges on it.
