When Stannis originally got back to Dragonstone from Kings Landing, he could of sent a Raven to Eddard in Winterfell informing him about Robert's children, and Eddard could of warned Robert when he arrived. But Stannis was to upset about Eddard being named Hand of the King in his stead, so he just wrote that opportunity off. However, Stannis had no problem breaking out the Ravens to explain things to the rest of the realm when it came time for him to "Make his Claim". In my opinon, Stannis doesn't deserve the Iron Throne. He left his family out to dry after Robert slighted him. From what we have seen, he didn't take every opportunity to warn Robert about his children and his enemies, and he killed his little brother Renly with an evil Shadow.
In Stannis own words, "The Good Act does not wash out the Bad, nor the Bad the Good". He allied with this sorceress Melisandre to try and take his throne, and she has been performing some pretty dark arts and magics along the way. Whatever Melisandre's true intentions are we have yet to see, but in my opinion Stannis lost his right to the Iron Throne when he chose her over his own family. To prove he chose her over his own family, he was going to let Melisandre sacrifice Robert's bastard son Edric Storm just because he had Kings Blood. I hope Stannis kills Roose and Ramsay, and then dies a slow and horrible death himself.
So due to all the slights sent Stannis' way by his older brother, maybe Stannis subconciously wanted something to happen to Robert.
Edited by mattah84, 29 March 2013 - 12:40 PM.