Another example where the Duolingo English translation sounds very unnatural to a native speaker. Nobody would ever say or write this.
OK in a contrastive statement. "We are twelve children - but you are twelve adults." But I can't really imagine children using this rather stilted format. More likely would be, "We are children - there are twelve of us."
Is there any difference in sound between "Nous sommes deux enfants" and "Nous sommes douze enfants"?
There is, douze has the ou sound which is similar to "oo" in English, while deux has the eu sound, of which I do not know any equivalent.
I totally agree with the comments regarding this sentence. Only a non-native speaker of English would use this form.
why was dozen not acceptable for an earlier one (about cats, I think) for me when at the same time it was accepted for someone else
I think you're thinking of "douzaine", not "douze".
The hover hints show "There are ... (of us)" but "There are twelve children of us" is not accepted neither.
"Kid" is more like "gosse, gamin(e)" - a bit slangy. "Child" is "enfant".
My seven daughters and five sons would never identify themselves in this stilted way ;)