What does the man with the red eyes want from the children?

The man with the red eyes is a proxy for IT. IT wants to gain control of the children's minds so that it can bend their wills to its will. As the man with the red eyes puts it, speaking for IT:

For you, as well as for the rest of all the happy, useful people on this planet, I, in my own strength, am willing to assume all the pain, all the responsibility, all the burdens of thought and decision.

As the children find out when Charles Wallace's mind is taken over by IT, this process of mind control is dehumanizing. Charles Wallace becomes a puppet of IT. IT is a disembodied, oversized brain that controls the entire population on Camazotz. IT forces rigid conformity on all the people and severely punishes any deviation from the norms it has set up. The people of the planet no longer have any free will. Charles Wallace himself becomes cold and heartless. IT represents evil, and the children must battle against it.