Was there another very similar film made in the 1960's?

I saw this film recently and enjoyed it. It's very real, exploring an advanced concept - alien invasion via an energy transmission which manipulates human genes to create the invaders. All this in 1960!

Having watched the film, however, I seem to recall seeing another which was very like it. No, not the 1995 film with Christopher Reeeve. This one dated from the 1960's (I think) and I remember one chilling moment very clearly. One of the principal characters asked the leader of the children "What do you want?". In a creepy, emothionless voice the child replied "We don't know".

Does anyone know what movie I am thinking of? I would very much like to watch it again, so any help would be appreciated.

Thanks.