The 7th prohibition is that we are forbidden from handing over some of our children to the idol Molech, which was well known in the time when the Torah was given.
The source of this commandment is G‑d's statement (exalted be He), "Do not give some of your children to be passed over [a fire] to Molech."
The Seventh chapter of Sanhedrin explains the way in which this idol was worshipped: [They would] light a fire and increase it, and he would take some of his children and hand them over to those who took care of serving this idol, and he would pass him over the fire from one side to the other.
The prohibition on this act is repeated in the verse, "There shall not be found among you anyone who passes his son or daughter in fire."
One who transgresses this prohibition intentionally is executed by stoning; if he is not executed by stoning, he is punished by karet; and if he did so unintentionally, he must bring a sin-offering.
The details of this mitzvah have been explained in the seventh chapter of Sanhedrin.