Try a sort of 'create your own' type thing. My two love this. I give them tortilla wraps, bits of spicy chicken, burgers, meat skewers etc (you could barbecue that part) also bowls of salad cut up small, grated cheese, mayo, ketchup anything. Then let them put together the combination they fancy.
Fruit skewers are easy and also go down well too. You could even barbecue those too, if you felt like it!
Lastly, if you're outside, don't forget ice-cream, but buy cornets. Take it from me - ice cream in a cornet and they'll worship you forever. I have the boys' friends to tea all the time, and they ALWAYS ask if I've got cornets! Dead cheap, very impressive to small people. Trust me!
Have fun xxx