I wanted to watch Chaos;Child, but I heard I needed to watch Chaos;Head, which is a prequel, first. Is this true?
Child is a sequel, I haven't watched all episodes (yet) from the second season of Chaos, but by skipping Head you'd miss out on a lot information that happened during certain events. Those events and the way they happened as well as the thoughts behind them will definitely make Child easier to understand. And yes it is pretty boring the first few episodes but it is a nice complex plot-twist compared to your average anime.
Both shows are quite confusing at first. Chaos Child is six years later after Chaos Head. The NewGen events in Head has connection to Child as well as about turning delusions to reality. The characters from Head will have no role in Child so don't wonder where they are. They would have very likely have left the city and went to live somewhere else since it was six years.