The attacking order of creatures is decided by when they came into play. The creatures that were summoned earlier will attack sooner than ones recently put into play.
That is incorrect. There is a set order depending on the position on the field. The exact middle card attacks first and so on with the order going with the order of when they would come into play but any disturbances to the order or replacements are not considered, i.e., if your middle creature dies and and you play a new creature (which would take that spot), that new creature will attack first even though you have other creatures that have come into play earlier.