Sounds like so much fun.
Bug cupcakes sound like a fantastic idea.
Hanging some handmade bug/butterflies from under the verandah with balloons for decorations. Make a few big ones from some coloured cardboard from the papershop for the littlies to paint or decorate.

If you have any other bigger kids coming they could go on a bug/butterfly hunt if they would find any on your 4 acres (sounds like heaven BTW) with some supervising adults of course

Depending upon numbers you could even get some of those bug containers and use them as party treat containers? Or put a magnifying glass in each treat bag for them to check out little bugs at home?

Have you seen a website called Butterflyadventures? They are australian and sell Butterfly Chrysalis Kits. Check them out.. It could be a awesome table centrepiece and something for the little people to gaze at. I just went and checked their website by googling for butterfly adventures, and they do kids parties in Melbourne, not sure where you are though??

If you are doing the invites yourself, could you do them in the shape of Butterflies and Bugs maybe? You could do Butteflies for girls and Bugs for boys if you wanted....