Kill two birds with one stone - invite your family and friends over together with the wee ones friends and their parents - hire a bouncy castle do some basic grub and ask people to bring a bottle and make a day of it. Don't have an indoor party in the summer. This will work out far cheaper, the kids will have a ball and you have plenty of people on hand to supervise the play - you can always state a tie limit on the invitations for the kids and their parents and keep family and friends on to carry on the party and to tidy up after the littlies have gone.
For food do some basic sarnies, sausage rolls and fairy cakes, couple of big bowls of crisps and get a tub of icrecream and do cones with sprinkles for afters.
Probably too little for a pinyata, but you could put some wrapped sweeties inside baloons then blow them up and let the kids pop them to get the sweeties inside.
I guarantee that the jones will be trying to keep up with you.