I did a lady beetle theme for my daughters firt birthday and had beetle invites complete with googly eyes and antanae.. I had bright green tablecloths & napkins (grass) and orange, yellow and pink balloons (bright flowers) which I actually twisted into flower shapes.. I cut smallish circles out of red cardboard and drew on spots with a marker to look like lady beetles then I blu-tacked them over the walls and down the corridor at almost kiddie height in a trail.. Also I found wind-up lady beetles at the party shop (which I also saw at spotlight) which entertained some of the kids for a while..

I would keep it simple, balloons and music with a few toys - you can always gauge how the party is going and bring more stuff out should you need it, although one large toy (like a plastic cubyhouse - you can get cheap ones at bunnings) which you can bring indoors will keep more then one child amused at a time (you don't want kids having "sharing" issues on the day) - you could put some little plastic tea sets inside the cubyhouse with some dolls and you will be suprised how little boys can really go for that type of pretend play.. or at least opening and closing teh door/windows

If the kids start to gat narky then bring out little "lolly bags" or little bags with something small for each child and that will get their attention for a while as they open up *their* little gift (like the wind-up beetle, streachy balls, plastic insects/snakes etc that you can get at the dollar shop or kmart)