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.

In Freak the Mighty, why does Max attend the fireworks without Grim and Gram?

Although Max has been allowed to go to the millpond on his own for a few years, Grim and Gram don't let him go alone on the Fourth of July. On the Fourth of July, the fireworks are set off by the millpond and many people attend.

This year is different. This year, Max does not have to go with Grim and Gram because he is going with Freak instead.

Gram says the two of them going together will be good because "she's afraid he'll get crushed or something." Max doesn't end up preventing Freak from being trampled, but when the punk Tony D. starts accosting them, Freak is able to climb on Max's shoulders. Later, the police officers call Max a hero for saving Freak, which Max thinks is weird because he feels like Freak's brain saved them. Freak and Max going to the fireworks together is good because they are able to protect each other.

It's also worth considering that Max does not have friends, so Gram is probably happy that this year Max has a friend to go with instead of his grandparents.