How many people should I overbook for an event?

I am helping to plan an event at work. It is a holiday party for 375 homeless children (including parents, 525 people total). In the past, we have had a hard time meeting our 375 target. How much should we over-invite our clients/families in order to meet our goal? Last year we invited 260 parents and 496 kids. In the end, we had 230 families and a total attendance of 385 people.