BBC | South East Wales
Stuart Bates, 43, and his son Fraser were struck on the A4119 at Talbot Green at about 00:30 GMT on Sunday.
A 22-year-old driver of an Alfa Romeo has been arrested and bailed.
Wife and mother Anna-Louise Bates wrote on Facebook that her "amazing fab little boy" had donated all his organs to transplant patients.
The pair were hit while crossing the carriageway near the Arthur Llewellyn Jenkins furniture store near Llantrisant.
Mr Bates, from Llanishen, Cardiff, an IT programme manager for PHS in Caerphilly, died at the Royal Glamorgan Hospital shortly after the crash.
Fraser, a pupil at Llysfaen Primary School in Cardiff, died at the Bristol Children's Hospital on Sunday night.
Mrs Bates, 39, who nicknamed her son "Fraser Bear", said in an earlier Facebook post: "Am waiting for the organ donation people to arrive to hope that I can pass on some bear love at least to someone else." Continue reading.