You could do books. There are always books in the $1 of Target. Or go to Half Price Books and pick out a variety. Or, if you have time, order a bunch of cheaper books from your kids' book order that comes home every month. You may even be able to go into Scholastic.com if you already have a password and order some stuff on your own, not sure exactly though, but I always get emails from them about getting this or that.
$5/kid is pretty darn cheap, so if you have the funds, you could do a $10 gift card to Target or B&N.
Marshalls or TJ Maxx was a good suggestion too. They have really good toys for pretty cheap. You could always take a look and get some puzzles, CDs, or books from there, or even action figure type toys.
Or you could just get helium balloons and tie a sucker to each one and have the kids take that home.
Also, my kids love those glow in the dark wands/bracelets/necklaces and we've gotten those for party favors before.
Oh and as far as siblings...did you invite them? If they are invited then yes of course, if you didn't specifically invite someone then I hope they don't show up! If you just mentioned that siblings were welcome, then yeah, I'd have something on hand.