So if my baby is size 6m regular and 9/12m Jewish I should get a 0-1m size?!
I can never buy my newborns things from there because the smallest size is too big, so I think you'd have to try that and be prepared to return. Or buy 2 sizes.
I bought my 2.5 year old two dresses Chanukah time last year when she was about to turn 2 in the 18 m - 2t size. They'll be fitting her now for succos.
How does your 1yo wear 4-6? My petite 4 month old is wearing that size and it is slightly big but 2-4 months wasn't fitting anymore...
I don't find that they run so big. Maybe a bit bigger than American sizes but not a full size bigger. Maybe half a size bigger. It also depends on the clothes. I don't find them to be so consistent in sizing and some items will be larger than others. Their pants in the baby section run big since they are made the same proportion as the shirts but babies tend to have shorter legs proportionately...