Anyone can discreetly report a very concerning situation involving the circumstances of children to their local Child Protective Services (CPS or in some places, Child Welfare Services) agency. Once a report is made, the agency will look into the facts and details and go from there.
It's definitely a discretionary decision and as others have stated, there are various things to consider. However, it's also important to note that children do not have the ability to truly advocate for themselves nor understand the complexity or danger of difficult family situations.
In my opinion, it is better to report significant causes for concern and let CPS handle the rest, than to not. It is also my opinion (and some experience) that a report to CPS does not automatically denote removal of children from the home. There are many factors to consider including what is best for the child(ren) and in some cases, it may just be that a child's parents must attend parenting classes or whichever interventions are deemed necessary. It very much depends on the situation. If anything, one can call their local CPS office and speak with someone regarding their concerns.
*On a closing note, there are people that are obligated to report ("mandated reporters") and include teachers, doctors, dentists, nurses, police officers, and so forth and so on. State websites typically provide this specific information.