Birthday Help!

At that age they don't need much, they just like to play.
Have some easy, cheap games, like pin the tail on the donkey, or a scavenger hunt in the yard, maybe a pinata (the loot becomes an instant goody bag!) Set up a game shooting targets with water or nerf guns. Make paper airplanes and see who's goes the farthest. Let them decorate cookies or cupcakes. No need to feed them a bunch of food, have some pretzels and fruit out in case they get hungry while they're waiting for cake and ice cream.
In short, keep it simple and sweet :)

You could do a bowling or roller skating party. Order some pizza and bring a birthday cake.

Kids this age like the traditional party games that were popular when we were kids. Pin the Tail on the Donkey, Hot Potato, Musical Chairs, Freeze Dance/Freeze Tag, etc. You can have a really wonderful party at home. Keep it simple by holding it in the afternoon (2:00 - 4:00) and just serve cake and ice cream. You don't need more food or anything else.

A friend had a party at Burger King. The kids got solo access to the play area. They set up a dedicated register, if you were with the party, you just placed your order for what you wanted and the party hosts picked up the tab. I don't know if there was a base price, but food wise @15 - 20 people was $90 in food / drinks.

First you gotta decide on a budget.
Then from there, decide on how many to invite. Because how many you invite, directly affects the cost of the food/cake size/goody bags amount etc.
Then from there, decide on where and when.
If having a party at a meal time, then you gotta also pay for lunch, for them and the food. Versus, if it is at a non-meal time, then you just provide cake/ice cream, drinks etc.

Per our kids' parties, we only invite their friends. Not an entire class. Even my kids have no desire to invite an entire class. Just their friends.
Under 8 kids.
We also discuss with them about "budget." They fully understand.

My son turned 6 recently, and we had a budget, he invited 5 friends (his closest buddies), he wanted a Pinata, a scavenger hunt, we had racing type games, egg on a spoon games, a "guess how many are in the jar" games and the winner gets to keep the jar, we held the party at a nearby park, thus no fees for that, we had a cake and food which I got from a nearby place that was good and affordable, the boys had TONS of fun! Because it was at a park and boys LOVE to run around. We even took them hiking because the park is a hiking type park with tables, and we brought my son's bike there too, which all the boys had fun riding! And they also just played make-believe with their own made up games, collected sticks and stones and chased each other around.

It was all a ton of fun for them, nothing had to be rented or paid a fee for, and they all had a blast! My son was super happy and the boys all talked about it for days!

Kids that go to school together and don't see eachother outside of school LOVE to be together. You can plan some things but honestly they will just enjoy playing with one another. Lots of organized games will make it not as fun. Some things I have done are obstacle course, hide and seek, capture the flag, scavenger hunt to find candy and other prizes that I have hidden, nerf gun battles. There are a lot of children's party ideas online so try searching it.

How about a pirate themed party with a scavenger hunt and gold chocolate coins as the treasure? You can buy a cheap set of face painting crayons and do eye patches and mustaches for all the boys yourself. When we do a party at home we just get out all the bikes and outside play stuff (basketballs, mini golf set, etc) and let them run amuck. They run around like crazy people and have a great time, no real activity is needed.

How about holding it at a local park with a playground? That way you have built-in entertainment at minimal cost.

I had both of my boys birthday parties at a go-cart race track with bumper boats and a mini-golf course. Do you have something like that in your area? Ours charges $10/kid for a birthday party, and they get three activities. We did a game of mini-golf, a go cart ride and a bumper boat ride, and I brought drinks, snacks and homemade cupcakes. Last year we had them at the bowling alley also for $10/kid, and that included pizza and drinks.

Is it past mid October? You could do a costume party?
buy some mini pumpkins and they could paint/decorate them.