This is the 14316th most frequent Russian word.
отдельная
"Separate" or "individual."
The word "отдельная" here is used to describe the room as being isolated or independent from the others.
Эта комната отдельная от остальных.
This room is separate from the others.
In this sentence, "отдельную" refers to an individual or specially allocated portion meant for the child.
Мы заказали отдельную порцию для ребёнка.
We ordered a separate portion for the child.
Here, "отдельная" signifies particular or unique attention or gratitude.