travel to taichung & taipei in 1st - 6th dec

Hi..I read the forecast it mentioned dec will be very cold from 15° to 22°..is t shirts n skirts enough? Do we need to bring sweater or winter wear? We are bringin our parents age between 57-63 and 3kids aged 7,9 and 1 infant abt 1year 3 months at that time..thanks!