Kids around that age tend to not be too fussy,
little sandwiches cut into triangles (umm fillings you could just stick to the basics, ham, chesse - just get the plastic stuff, they'll turn their noses up at heaven forbid real cheese
, tuna mayo tends to get eaten, throw in some jam ones if you dare - sticky!!)
mini sausage rolls,
cocktail sausages,
some crackers or breadsticks
mini pizzas are usually a favourite, i usually just get some tesco value etc ones and cut them into small squares, just stick to cheese and tomato if you want it to all get eaten (you can make them before and serve cold).
Salad/veggie sticks - cucumber, carrot, celery and maybe a simple dip like sour cream and chive.
Fruit - pickable things like berries, grapes etc are best as they wont spoil if you pop them out for them.
if you trust them not get get things all over your carpets etc (or if you can throw them outside) maybe some ice pop type things as it's pretty hot.
Hope i've given you a few bits to go on there hun!