Hello, wouldn't 'We promise not too tell.' be 'Obiecujemy nie powiedzieć'?
Well, the first one is too different (Obiecujemy, że nie powiemy), and I understand in your second comment you're against the comma splice... well, I'm not sure if I ever encountered someone having a problem with using comma splice in Polish. And we don't want to split the English sentence into two if there was just one in Polish.
But in the English translation, the meaning is quite the same regardless of presentation English being much more fluid.
I'm in favour of rephrasing because I find the sentence in Polish confusing.
At first I thought the speaker was saying "We won't say" (something you are asking us to divulge). (Instead; since we refuse to tell you that thing) "we promise." It's ambiguous.
I vote for changing it to "Obiecujemy że nie powiemy".