I think the best bet would be to go for the sibling Sarah. Her baptism is 28 May 1848: Sarah Robson d/o John and Mary Robson, labourer, Todridge.
Although Ridley's baptism may say Sept 21, it is out of place, and Sept 21 was not a Sunday and Oct 21 was. The entry before him was 21 Oct and the next was in November. Does it say Sept in the actual parish record or just in the BT's? If it was Oct then that Male child Robson may be him but unamed as of the registration. And his baptism said parents were John and Mary.
Janis