After some very natural decorations that don't cost a thing?
I'm sure your kids will love to help out with this project.
Pop out into your garden or volunteer to clean up your local school or neighborhood park, even raid a relatives yard. Find some dead branches hopefully with spider webs already attached but if not, never fear, I'm sure you have a spider web or two around your eaves or gutters that needs to be grabbed. Pop on some nasty looking plastic bugs & spiders which I'm sure anybody with boys has ample of. Pop the branches into vases or place them along windsills and on tables.
N.B. Try to find a nice new home for any spider you come across if you love critters. If you are like me though and don't but want an authentic look, just spray them. I'm almost scared to type that for fear of the wrath that may be brought up on me by spider lovers so in advance, I'm sorry, I'm just not a spider lover. At all.