Your explanation resembles Sprite by Ishikawa Yugo as follows. These links to Wikipedia provide basic information on the respective subjects although unfortunately the plot of the manga isn't explained in detail.
I am looking for a manga
Sprite is a manga
last two floors of a skyscraper get teleported through time.
Not sure if top two, but residents (including a thug) on high floor of tall building are subject to phenomenon, and on the roof-top are group of children
There is a group of kids that can see time as a black tide and don't age if they escape from it.
"Lost children" (although they've "lived" longer than any of us) are dealing with the black liquid that has affected time.