Even if the weather isn't fine - still go for picnic blackets and cushions, but inside. And the blankets will protect your carpets.
I like the little box ideas - a bit like the pick and mix boxes you get in cafes.
I'd go for something like:
a buttered bread roll or pitta pocket,
a lump of cheese,
a little ham/chicken,
a cocktail sausage,
a couple of sticks of carrot/cucumber/pepper/tomato,
a box of raisins,
a carton of juice,
a yogurt,
a biscuit,
And a napkin to eat it off
We aren't big sandwich fans here - hence it being seperate.
Also check any diet issues - we ahve some friend's children who don't eat pork plus some veggies. Ask for people to RSVP with this information included.
Or - and I am sure children would love the idea, include an RSVP menu choice list with the invite. Give then a choice of sandwich (say list of 3), a piece of fruit and a flovour jiuce. And then when each child RSVPs you can label up a box for each.
Take cake home on party bags.