A general remark, which might be helpful. In metrical Europa all sizes of hats are always measured by the contour of a head. S 35 means that the hat is meant for a head with a contour of 35 cm. This is quite an extraordinary small size, even for a child of 10-12 years old.
Being a hat wearer for years, equipped with a rather small head, I always have to buy in the hat shops the smallest size available for adults, S 52 or S 54. A lower size than S 52 is not or hardly available. So, it might be possible that this helmet looks smaller than you are used to and the "35" has notihng to do with the size?