It's quite colloquial to say "I think we all ought to cut them some slack".Well, yes, it does mean 'a bit more slack' but if these were British parents talking in an everyday situation, I think they might say: "I don't think we should be too strict with each other". I find it difficult to translate the sentence more literally in a way that British parents would probably say in that situation.
You may well be right As I may have mentioned before, I'm a bit out of touch with more modern expressions, having left the UK in 1971. According to The Free Dictionary, the term emerged in the mid-1900s (by which I presume they mean about 1950) and they call it 'N. American colloquial', so I imagine that by the time it became common in the UK, I'd already left.It's quite colloquial to say "I think we all ought to cut them some slack".