As a temporal adverb, I've always understood ещё to mean still. Therefore I don't understand its function in statements of the type:
Здание, в котором ещё вчера находились дети...
Same. In this particular sentence you would translate it ... only yesterday ... , but essentially the meaning is the same: yesterday they were still there. Although, there is a subtle difference in that it is вчера that is stressed, so only yesterday is a better fit.
Вчера ещё в глаза глядел,
А нынче - всё косится в сторону!
Вчера ещё до птиц сидел, -
Все жаворонки нынче - вороны!
Марина Цветаева.
Only yesterday (the meaning is emphatic ( quite recently ) and often opposed to today's changed situation.
The word ещё can be used to emphasize something, so here it's almost the same as только вчера. You may even skip it without losing sense.
You can translate it this way:
The building where yesterday there were still some kids.