How about The First Men by Howard Fast. Sci Fi short published in 1960 in The Magazine of Fantasy and Science Fiction. Text is now freely available online - Full Text
The excerpt below from the text is what makes me think it's the story you're interested in. It tracks quite closely to your reference of them placing themselves one second in the future.
We got the three years we needed, and as for the gray barrier that surrounds us, the children tell me that it is a simple device indeed. As nearly as I can understand, they altered the time sequence of the entire reservation. Not much -- by less than one ten thousandth of a second. But the result is that your world outside exists this tiny fraction of a second in the future. The same sun shines on us, the same winds blow, and from inside the barrier, we see your world unaltered. But you cannot see us. When you look at us, the present of our existence has not yet come into being -- and instead there is nothing, no space, no heat, no light, only the impenetrable wall of non-existence.