That every employer shall keep posted in a conspicuous place near the principal entrance, in any establishment where children under sixteen years of age are employed, permitted or suffered to work, a notice stating the maximum number of hours such child may be required, or permitted to work, on each day of the week, the hours of commencing and stopping work and the hours allowed for dinner or other meals. The form for such notice shall be furnished by the state labor commissioner, and the employment of any child for a longer time in any day than so stated, or at any time other than as stated in said notice, shall be deemed a violation of the provisions of this act.