temporary work

Hi.  I work 3x days per week but have decided to hand in my notice and be full time mum as have 3 children, including young baby and are trying for a 4th baby.  Hubby works full time.  Just wondered my work have asked me if I will fill in temporarily if anyone off sick or on holiday.  Do I need to declare this to the tax credit office??  I wouldn't know when I would be working??