
There's a strap for each shoulder, coming together into a single buckle in the front that passes through the child's legs. The issue we've run into is that when the straps are adjusted short enough to fit my son's shoulders (he's 21 months), there's not enough room in the "loop" for his helmet to fit through.

Enough was enough, it was time for a new strap. Browsing through strapworks.com, I found what I was looking for. A split release buckle would let me separate the shoulder straps. The factory belts themselves had always seemed a little stiff, so I opted for 1" seatbelt webbing for my version. And a strap adjuster would serve just as its name implied.
Now, if you were crafty enough, it would be a piece of cake to order the pieces and sew them yourself, but sewing isn't my craft of choice. So I spoke to David in the custom department at Strapworks. After faxing in a waiver that I wouldn't hold them responsible if anything bad happened, he gave me a great quote for the entire finished set. Each shoulder strap would be stitched to a strap adjuster, and the leg strap would be sewn to the bottom of the split buckle.
Three days after my order (they're not too far away in Eugene, Oregon), the straps arrived. I had ordered them extra-long so that I could cut them to size, but if you order your own and want to match the dimensions of the existing system, the shoulder straps need 30" of webbing and the leg strap needs just 8" (total length after sewing - add maybe an inch-and-a-half to loop back and sew). You could even order just the split-release buckle and its strap, and re-use the existing shoulder straps but threaded through the new buckles.
I removed the old straps and used them as a guide to cut the new ones to length, melted the ends on an iron to keep them from fraying, then melted a hole through the webbing with a soldering iron for the bolt hole. The new straps went right on without a problem. If I hadn't been taking pictures, it would have been around 5 minutes to do the swap.
Here's a comparison of the new leg strap (on top), and the old one:


