Child seat with a height adjustment device for a headrest , the headrest forming an upper head portion of a backrest of the child seat for motor vehicles, the head portion being displaceably arranged with respect to a lower fixed portion of the backrest, wherein the head portion is guided at the lower fixed portion of the backrest within a positive guide , a locking device being provided for locking the displacement of the head portion with respect to the lower fixed portion of the backrest, wherein the locking device being formed by an actuation unit positioned at the head portion and by locking members cooperating with the locking device , wherein the locking members are positively guided perpendicularly to the displacement of the head portion allowed by positive guide towards the lower fixed portion of the backrest in the head portion and may thus engage with recesses positioned along the positive guide within the lower fixed portion of the backrest and may lock a displacement of the head portion with respect to the lower portion , wherein the locking members is guided at the actuation unit within a positive locking member guide , the positive locking member guide of a locking member is formed by a respective channel extending obliquely to the possible direction of motion of the head portion and obliquely to the possible direction of motion of the locking element , which an extension on the locking element engages in, the channel of the positive locking member guide is formed with plane-parallel opposite surfaces as inclines, where the extensions being formed as runners having, at the ends of the locking elements , plane-parallel counter-surfaces corresponding to said surfaces are able to glide form-fittingly within the positive locking member guide . wherein the head portion is guided at the lower fixed portion of the backrest within a positive guide , a locking device being provided for locking the displacement of the head portion with respect to the lower fixed portion of the backrest, wherein the locking device being formed by an actuation unit positioned at the head portion and by locking members cooperating with the locking device , wherein the locking members are positively guided perpendicularly to the displacement of the head portion allowed by positive guide towards the lower fixed portion of the backrest in the head portion and may thus engage with recesses positioned along the positive guide within the lower fixed portion of the backrest and may lock a displacement of the head portion with respect to the lower portion , wherein the locking members is guided at the actuation unit within a positive locking member guide , the positive locking member guide of a locking member is formed by a respective channel extending obliquely to the possible direction of motion of the head portion and obliquely to the possible direction of motion of the locking element , which an extension on the locking element engages in, the channel of the positive locking member guide is formed with plane-parallel opposite surfaces as inclines, where the extensions being formed as runners having, at the ends of the locking elements , plane-parallel counter-surfaces corresponding to said surfaces are able to glide form-fittingly within the positive locking member guide . <EOS>