日本語 Japanese Ask Questions Thread 4

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Japanese Ask Questions Thread2
http://dis.4chan.org/read/lang/1206158123/1-40

Japanese Ask Questions Thread3
http://dis.4chan.org/read/lang/1267485093/1-40

Hello all!
So i`ve come across this だけで particle.

I know だけ　means `ONLY`, but why is there a で particle?

Example:

私は子供が好きだから
子供たちと一緒にいるだけで十分よ。

I like kids, so, only being with them is enough(for me)?

Is it used as instrumental case?