Max has never been allowed to attend the town's Fourth of July fireworks display without Grim and Gram before, but this year is different. Max will be going with Freak, and the two boys will be allowed to go together since Gram is
... afraid he'll [Freak] get crushed or something, she actually thinks people are going to step on him... (Chapter 6)
Max has been allowed to play down by the millpond for years, where they shoot off the fireworks, so he's "never understood" why the Fourth of July should be any different. But Grim and Gram also recognize that Max is getting older--he and Freak will enter the eighth grade together at the end of the summer--and they believe he is old enough--and big enough--to take care of himself and his little friend.