I think it's wonderful you're inviting the siblings of friends. I like Nina's answer very much.
Do you have any group games you can borrow from a friend such as Pictionary? Those kinds of games are fun for everyone and the younger siblings will feel so important being invited to play along and they can come up with some really good drawings too !
Another food item that's easy is cooking marshmallows over a fire, even if you use just your grill with coals and everyone puts their marshmallow on a branch stick and roasts them and then you can all make s'mores which means you put the cooked marshmallow between two graham crackers with a tiny piece of Hershey's kiss chocolate and squeeze it all together! Everyone would love that.
Also, ask everyone to bring a snack like drinks or chips and fruit or cookies. . That way you don't have to come up with anything except paper cups and plates and napkins and you can even ask people to bring those . People really like to bring things to parties.
One of the cutest ideas I saw was when everyone decorated their own donuts. The donuts were bought plain and then the icing was made in a big bowl from confectioners sugar, which your mom may have , and a tiny bit of milk and a spoon of vanilla.. Be careful not to add too much milk-it only takes a TINY BIT of milk to make icing with a whole bag of confectioners sugar (called powdered sugar).
Stir it to make a white icing that everyone can spoon on their donut and then have chocolate chips, sprinkles, and crushed cookies such as Oreos to sprinkle on top and have each topping in separate bowls to choose from . That way it can be considered the birthday cake and everyone decorates their own donut. You can even ask people to bring things to decorate the donuts with.
I hope you have a good time. If you live in the USA let me know and I may be able to give you ideas where things are very cheap to buy.
