If you are like me, you can easily go over board, and over budget. Here are a few of the places i go to help me keep on budget.
Dollar Tree and 99 cent store
your Downtown wholesale district
All of these stores have tons of things you can get for under 5.00 (some have everything for 1.00)
you can also find a lot of things that are versatile and re-useable. i have a cabinet of party supplies i use almost every party or event i throw, and some of them I only paid 1.00 for, but i have probably used 15 or 20 times already.
the other resource you have is your home! i am constantly using things from around my house to decorate for parties and events, and often times vases or other decorative pieces i use in a party will find a permanent home in my bedroom or living room. so take a look around your house and see what hidden gems you may have lying around, picture frames, plants, vases, statues can all come in handy. is it themed after your child's favorite tv show? i bet he or she has some toys, or stuffed animals or posters of the character, all of that can be put to use.
just try and think outside the box, and you will have a great time decorating for your party.