I know that you said the children don't watch much TV, but you can still get a lot of ideas, printables, etc from www.dltk.com website, as well as others. When my twins were 4, I did a princess themed party where we made our own medieval style hats with flowing fabric (used solid color bday hats & threaded cheap sheer type material thru the end). I made a castle cake (which I saw in a magazine -- still have the picture/pattern for it). You could even include your son (king or prince) and his "knights" (male friends). Make cardboard swords & shields and capes from just rectangles (of $1/yd material from Walmart) held together with 1" elastic strips & safety pins (use elastic to prevent choking if someone pulls on the back of another's cape).
That's just a few ideas from a party I did...there are LOTS more on the web. Just decide on a theme -- princess, "ocean/beach fun", "farm fun", "cowboys", etc. and do a web search on "children's parties" + whatever theme you decide.
If you have a Hobby Lobby close by, they often have their t-shirts on sale half off which would be 1.50 each. You could do a treasure hunt in the yard and let the children paint their own treasure chest and decorate them. They sell these small wooden boxes at Walmart for about 1.50 each. They are plain unstained wood and have a closing clasp on them. You can also hide treasures in a dough type ball made of coffee grounds and flour. I seen it yesterday on PBS, but they did not give exact measurements on the recipe. When dry, they look like a coconut. They are easy to break open. To make sure each child had one, you could label them before you hide them. You could easily make a treasure chest cake. Last nite while waiting in line at the grocery store, I thumbed thru a Better Homes magazine that had birthday ideas and the treasure chest cake was one of them. Good luck!
How about a "color" theme. Do a RED party this year, blue next, etc. By the time you run out of easy to do colors they'll be old enough to want their own parties with their school friends.
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