Q:Did Seth and Nephtys have any children?
No, Seth and Nephthys did not have any children. Anubis was the son of Nephthys and Osirus.
Q:A site that i was looking at said that atum coughed up shu and tefnut. can someone plz explain that?
This is known as the Helipolitan (Iunu) creation myth. This myth about the origin of Shu and Tefnut states that once upon a time the god Atum went to the city of Annu and he created Shu and Tefnut from his own body through masterbation. In the story of the creation, Atum is described as saying: "I had union with my hand, and I embraced my shadow in a love embrace; I poured seed into my own mouth and I sent forth from myself issue in the form of the gods Shu and Tefnut." Some versions claim he sneezed out Shu and spat out Tefnut. Later myths called this story into doubt and claimed that Atum's shadow acted the part of his wife; still others state that the goddess Iusaaset was Atum's wife.From Shu and Tefnut came Nut and Geb who produced Osirus, Isis, Nephthys, and Seth. These 9 God's are known as the Ennead.
"A few words of clarification for our Western minds. It may seem very rude and immoral the way Atum 'copulated with his hand', i.e. masturbated, but the ancient Egyptians seem to have had a rather down-to earth, practical view of the 'facts of life'. To them it was nothing shameful about the creator´s act, since he embraced the male as well as the femal principle, this was a practical way of explaining how one god could produce two more. As you can see, the quotation also gives an alternate way; he says he 'spat out' Shu and 'sneezed out Tefnut." http://www.geocities.com/nefertiabt/Iunu2.html
With that said, the title of 'God's Hand' was an alternative title for God's Wife, which refers to the hand with which Atum masterbated with to create Shu and Tefnut. The word hand is feminine in ancient Egypt and was easily personified as a goddess. So, it is thopught that the titles God's Wife and Go'd Hand had some sort of sexual connotations. Perhaps the meaning was to stimulate the creator God sexually (by ritual?) so that he would keep his fertiltiy and thereby cause the world to be recreated. While we don't know for sure what this titled entailed, we do know that sexual activities were forbidden in the temple or on temple grounds.