This is mentioned in Anne Applebaum's book on the Holodomor (Ukrainian famine of 1932-33), Red Famine: Stalin's War on Ukraine (2017). She notes (on pages 256-261) that many survivors claimed to have witnessed cannibalism throughout Ukraine, with a mix of those who heard it second-hand, and those who claim to have seen it personally.

In terms of the latter, the accounts Applebaum provides don't have the survivors literally seeing people kill and eat children, but instead they relay stories that someone admits they did it. One example has a father admit that after his wife died he killed first his daughter and then son to eat, and threatened neighbours if they alerted the authorities (they did anyways). A second has a woman admitting she killed her son to eat, and was also arrested. There is also reports from Sergio Gradenigo, the Italian consul in Kharkiv that parents brought their children to school themselves, afraid that if left alone the children would be captured and eaten.

As noted above people were arrested for cannibalism, which was largely seen as a repulsive act, even in the most trying of circumstances that Ukraine was dealing with. The authorities were also concerned that these stories would spread, which would be highly embarrassing, so efforts were made to keep them quiet; even so stories still got out, as we can see. Indeed figures rose throughout 1933: 9 cases were reported in March, 58 in April, 132 in May, and 221 in June that year. In the Kyiv region the OGPU (Soviet secret police) were receiving 10 or more reports a day of cannibalism, and other regions had comparable numbers, which Applebaum says "may have been serious underestimates." (p. 260). However as she notes, there was no official directive from the Soviet leadership in either Moscow or Kharkiv (which was the capital of Ukraine at this time) on how to deal with cannibalism, though she also admits that this may just not have been discovered yet (the Soviet archives are partially closed off still, especially anything related to the policing services).

With that all said, I need to add a few caveats to the above: the first, is that while Applebaum's book has received positive reviews, it is something to be read with reservations. Applebaum is not a historian per se, but a journalist, though she has written several books now. She is also openly political and strongly against Russia and Putin in particular, and has been a strong advocate of Ukraine since the events of 2014. This is in part because she is married to Radislaw Sikorski, who was the Polish Foreign Minister at the time and played a very key role in Poland's response to what was happening in Ukraine (Sikorski is now in the EU parliament). So while she may do a solid work of researching (which I'm going to touch on next), she also does have a clear bias to consider.

Next, Applebaum's research for this book has been noted for the fact that she didn't do any archival research herself. By this I mean that she was not the one sitting in the archives in Kyiv (or wherever) reading old Ukrainian documents, because she doesn't read Ukrainian. She had local assistants do this work for her, and while that is not an issue as a whole, this is not made clear in the book, and only came out after the fact. While assistants are of course used by scholars all the time, it is important to note this, as things can be misunderstood between the transmission of information.

I also need to note that the sources used for this particular section are largely from the testimony of survivors of the Holodomor who were interviewed by Harvard decades after the fact (I believe they started interviews after the Second World War, but think it may have been as late as the 1960s or even 1980s; I can't get a firm answer at the moment). The point is that they were recalling events from years past, after having experienced multiple traumatic events, which will easily distort memories. Applebaum also doesn't seem to directly be using the source material at Harvard, but a lot of her citations refer to testimonies sourced from third-party sources (other books, articles, etc), which further distorts the facts. This is further compounded knowing that the people interviewed were ones who were able to emigrate from the Soviet Union, doing so during the Cold War, so have both a reason to play up the atrocities of the Soviet Union.

So to give a short answer, there are reports that people ate their children, but it is hard to say for certain that the reports are true. It is quite possible it happened, and based on the evidence shown I would say it is very likely it happened on a few occasions.