I had the same question. It feels to me like an attempt at a twist ending, albeit an ambiguous one. Zellaby says the only limit to their power is that of the mind, so if they are so advanced, maybe they can live on as pure mind.
Or maybe they are off to reincarnate in the wombs of other women. We don't know how the Midwich women were impregnated in the first place, after all.
Another poster pointed out that the ending shot is not in the original script, so maybe it is just a gratuitous image for additional creepiness. As much as I like the basic premise, acting, and general eeriness of the movie, I found the storytelling to be rather rushed and clumsy throughout, so the confusing ending is not entirely out of place.
I agree. I thought the flow of the story was a bit sloppy too, but hey, it is only supposed to be a B-sci-fi flick. Fun movie though. A bit of trivia is that the actor who (SPOILER) outwits the children by thinking of a brick wall, appears years later in a Fawlty Towers episode as a con-man secretly hiding a brick in his briefcase.
"The books have nothing to say!" -- Fahrenheit 451