The reason why Primark was singled out was because
they made false claims as to the ethics of their clothing production. if you had watched the documentary you would have seen that Primark actually signed up to the ethical trading standards. Ergo the revelation that they are using child labour was obviously a breach of this
Also, the long and short of it is, if there was no demand for cheap bulk clothing OR if consumers came out in force to say, hang on, we will not accept this, then companies such a Primark,Gap and the rest would be forced to change their policies.
Of course Primark knew child labour was being used. They may not have outwardly sad "hey, hire some 7 year olds" but they are a large TNC not innocents abroad, and I would be extremely suprised if they did not know this was going on, especially as it was in India where this sort of thing is prolific.
If these children are able to stop sewing, they will not be forced to turn to prostitution. Nobody would be suggesting they simply lose their jobs- Perhaps the answer would be that the companies who employ these people pay the older workers more- meaning they do not have to more or less sell their children into sweatshop slavery, but that instead they can afford to keep them at school.There are already movements in india working along these lines
By shrugging our shoulders and saying "rather that than the alternative" no changes will ever be made, and we are, indirectly, condemning these children to a life of endless toil, mistreatment and low pay.
edit just remembered that in Nike's case, when they too were discovered to be using children as young as 3 to make trainers (it was John Pilger who brought this to light I believe) they had a
moral obligation to provide an alternative for the children once they stopped them working. What actually happened was that they paid the children to go to school until they were of such an age where they were allowed to work part time. They also compensated them for loss of earnings.