Party bag gifts do not need to be a considerable expense; there's a lot you can buy from a dollar store or Walmart. But remember, this is a celebration and should be fun, which goes for the goodie bag too.
Start by having basic party favors bags ready, then create a treat at the party the children can take home with them. This way, you can pop the DIY treat in at the end with a slice of cake, and you're good to go.
Pop in a sheet of stickers, make a personalized candy bar, or drop in some popping candy, a cheap notepad and pen, and a thank you note for attending and put them to one side, ready to add the final piece at the end.