Firstly, ask yourself some questions:
How much time will we be spending in the car?
How often do we expect to be eating out and at what kinds of restaurants?
What type of accommodation will we have? (Self catering/half board/all inclusive)
Will we be able to prepare any of our own meals?
What activities will we be doing? (Walking, swimming, water sports, cycling etc)
What opportunities for physical activity will we have? (Are there walking routes, sports facilities, areas to play etc)
What situations might tempt us to eat poorly? (Excursions with buffet food laid on, eating out with others, restricted menu etc)
Then, plan around your answers. For example, if you will be in the car for hours at a time, bring healthy snacks such as wholemeal bagels, oat or rice cakes, fresh fruit or vegetable sticks. Plan places to stop on driving days where you can have a healthy picnic and enjoy some activity, such as a game of Frisbee or bat and ball.
If you will be eating out a lot, choose grilled or broiled foods instead of ones that are fried or that come in a cream sauce. If you and your family order pizza, get it with toppings of vegetables instead of fatty meats, ask for half the cheese and ask for thin crust. You should be vigilant about portion control.
One last thought: All foods fit in moderation. The occasional splurge is OK, but be careful that your splurges are the exception rather than the rule.