How does Miller's assertion "there were no witches then" affect your view of the children in the play? Why does Miller tell us this outright?

Miller's assertion is, obviously, that the children were lying. They saw, in the witch trials, not only an opportunity to get back at those who they despised, but also as an opportunity to legitimately rebel against the harshly constrictive society in which they lived. The court gave them powers they had never had before. They could now abuse those powers with gusto, free from the persecution that was the lot of those whom they accused.
Furthermore, the witch trials also afforded them an opportunity to appease their guilt and transfer it to others. The girls had, in fact, performed rituals with Tituba and would have faced severe sanction if they had not pointed out that they were not entirely responsible but that others, mainly adults, were the ones who introduced them to, or forced them to, indulge in such wicked practices. When Tituba was blamed, she, to avoid persecution and torture, started blaming others, which opened the door for the anxious girls to follow suit. The girls played the blame game perfectly and their pernicious actions led to the arrest, incarceration, conviction and eventual execution of many innocents.
In addition, the girls' hands were strengthened because they enjoyed the patronage of the court and were practically freed from persecution. As long as they were unified in what they did, no one could question the veracity of their claims. They were beyond suspicion because they did not act as individuals, but as a group, and the court could not fathom why they would all share the same experiences if those experiences were not, in fact, real.
Added to that, the society in which they lived could not accept the fact that it had raised children who would do evil. If so, it was damned, and therefore it resisted the idea that there could even be the remotest possibility that the girls were acting on their own. There had to be an evil force that had corrupted them. The mindset was that the children were innocent and if they claimed that they had been influenced, it had to be true.
It was Salem society's refusal to acknowledge and accept its own shortcomings that further empowered the girls. They realised that they had free rein and abused the authority that they were granted. Since Salem was a theocracy, religious belief was the foremost test of its moral strength and the battle against turpitude. Acknowledging that it was flawed would be the death knell for its survival. Therein lies the irony, though, for it was exactly this belief that created the conflict and paranoia introduced by the girls.
Miller makes this outright assertion because he wants to emphasize the depth of the girls' corruption and their malice. Their behaviour was actually the tangible proof of how paranoid, anxious and corrupt Salem society must have been. A society which suppresses most of what is natural, condemns it, and limits the freedom of its members will, eventually, explode in anarchy once its members are given an opportunity to vent their frustrations and punish, as it were, those they feel are most responsible for their struggles. Typically ironic, though, is that they target the most vulnerable and not the ones who are, in fact, really responsible for their misery.