I called inquiring about kids BJJ belts. The guy on the phone was pleasant enough but didn't have a clue about sizing and thought the smallest size was a 0.
I realize that kids belts and gis are probably not a huge money maker for them but I think they should have a better idea of what they sell and what's available on the market. FWIW - kids sizes go down to 0000 which is considerably smaller than a 0.