Sci fi, horror, dystopian...? A bit of all of them.
This is a straightforward and somewhat leisurely story that touches on very deep and difficult themes, mostly indirectly, but explicitly in the last quarter.
Midwich is a sleepy English village in the late 1950s. One day, everyone in the village blacks out. They awake, apparently unharmed, only to discover that all the fertile women are pregnant - but the children they give birth to are not like other human children, and turn out to have extraordinary and disturbing powers.
It starts off by establishing the uneventful normality of the village. With dawning awareness of what has happened, most people indulge in denial and eventually a degree of acceptance. The abnormal becomes normal, and things get stranger still.
The big flaw of this book is its neglect of female characters, especially given that it is the women who are violated in such a profound way. More understandable is the overprotective attitudes of some of the men, exercising "benign censorship", especially for the less educated women. That may not be acceptable now, but surely typical of the period. It also oddly omits almost all mention of older and younger siblings of the Children (the capital C is used) and barely mentions the pain of the putative fathers.
The strength of the book is the way it raises so many philosophical issues in a relatively light way and barely 200 pages: fear of tabloid exploitation; the nature of self and individuality (and how it is affected by mind control and shared consciousness); whether scientific dogma overrides religious dogma; societal and biological pressures on mothers to bond with their babies; original sin; triumph over adversity and the desire to see good in situations; whether ends justify means; what it means to be human; evolution versus creationism; the nature of evil and what can be done in the name of self-preservation; the politics of colonisation and revolution. The ultimate question is whether humanitarianism trumps biological duty and hence whether civilisation could ultimately be our downfall in a hostile environment.
One of the problems Wyndham suffers nowadays is that to modern readers, his work can seem derivative, which is a dreadful injustice when in many cases it's because more modern writers have derived ideas from him.