There was a change to the Church Handbook. Here is an official Newsroom interview video with one of the Twelve Apostles, explaining some of the context behind the change. Additionally, the Church released an article about understanding the handbook changes.
Old answer:
I have not yet found an official statement from the Church to answer the question, but a few things to consider anyway.
Typically, the LDS Church is very quick to respond to controversial rumors or misinformation that gains popularity. For example, in September earlier this year, the Church issued a statement in response to the doomsday rumors about the "blood moon" that was to come. Such official statements are intended to set the record straight or answer questions when there is confusion or inquiries.
In contrast, this time, there hasn't (yet) been an official statement from the Church on the matter on their Newsroom website.
In addition, my roommate is a teacher at the Missionary Training Center and I'm told that the missionaries are being trained will be instructed by their individual mission presidents on the policies regarding joining the Church in same-sex marriage situations. (Correction = MTC doesn't teach policy; it teaches fundamental doctrine.)
Policy changes in such a large organization are not unusual, especially given dynamic political and social climates. In fact, half of what you quoted, this part:
The child would also have to commit to living the teachings of the LDS Church and disavow the practice of same-sex relationships and marriage.
is not new, from a doctrinal perspective. Being worthy for baptism and keeping the covenants entered into (i.e. "living the teachings" and not being part of groups or movements with teachings contrary to that of the gospel) have always been expectations for members of the Church.
So while the headline is based on authentic events, it has been sensationalized and poorly reported by news and social media. For example, your question title, "policy forbidding children of homosexual couples from joining" is worded like most headlines, but it's disingenuous since it makes it seem that they will never be able to be members of the Church. On the contrary, the policy only applies in very specific situations. In a letter to Church leaders, the First Presidency stated:
Our concern with respect to children is their current and future well-being and the harmony of their home environment. The provisions of Handbook 1, Section 16.13, that restrict priesthood ordinances for minors, apply only to those children whose primary residence is with a couple living in a same-gender marriage or similar relationship. As always, local leaders may request further guidance in particular instances when they have questions.
The intent of the policy is to preserve the harmony of the family in homes where Church doctrines conflict with the family's situation. From the article linked above:
In reality, very few same-sex couples would bring children for the formal Church ordinance of naming and blessing, since this creates a formal membership record. But Church leaders want to avoid putting little children in a potential tug-of-war between same-sex couples at home and teachings and activities at church.
This sensitivity to family circumstances is practiced elsewhere. For example, the Church doesn’t baptize minor children without parental consent, even if the children want to be associated with their LDS friends. A married man or woman isn’t baptized if the spouse objects. Missionaries don’t proselytize in most Muslim countries or in Israel, where there are particular sensitivities with family. In some African and other nations where polygamy is practiced, anyone whose parents practice polygamy needs special permission for baptism so they know that a practice that is culturally acceptable for many in the region is not acceptable in The Church of Jesus Christ of Latter-day Saints.
