When we went a few years ago we couldnt find anything for our kids, so my wife bought three of those small, soft-sided lunch boxes, and sewed waist belts onto them for the kids. They worked great, we still have them, but they are too small for the kids now. You might need to get creative like that.
We just got 3 fanny packs at Ocean State Job Lot, if you have one of those around, but they are adult sized. Only $4, and pretty good for that, but I don't know how small they go. They do fit our 12 yo DS, how is very slim.