First I would have said “Si l‘on n’était que douceur avec elle“. Then, you could change the sentence to “Si nous n’étions que douceur avec elle” without changing the meaning.

Literally translated, “if we were only softness with her“. Basically, the sentence means that they are not kind/soft with her.

If you want to use the adjective doux, you can say “Si nous n’étions que doux avec elle“.

I don’t know if it is a rule but it’s a common way of talking, however I’m only aware of it being used in the negative form:

• Tu n’es que méchanceté.
• Il n’est qu’amour et miséricorde.

This sentence expresses with emphasis how the subject is and only is something.

In this sentence, they are saying that if they were soft (and only soft) with her, she wouldn’t be that much motivated to catch up with them. One misunderstanding could be thinking it means if they were softer.