What are some good ways to take attendance at a large event? (8000+ people)?

I'm planning an event for the end of the summer and we are looking for ways to take attendance. This event is for young children (mostly preschool age) and their parents and runs all afternoon long. It is outdoors and we have two entrances (which people may be coming in and out of so clickers may not be that efficient). What ideas do you have that would be suitable for the age group and the scenario I described above? Thank you!