there are many "oxford" series for children,
最常有reading rainbow (~6 level),
oxford story tree (books + cd),
oxford reading tree (~9 level) each level have some "more stories"pack, read at home (5 level) 1-4 level, also have cd+book.
story tree可for小朋友自己睇, reading tree can be more interactive,(level 1, Only Picture) suppose parent/teacher reading with children; read at home同reading tree's format有點似, one story per book, (with cd), and also have some ex./activities.
In fact, story tree & reading tree stories' may have the same title,but the level may be diff. on the words/tense etc.