Should this be accepted as a valid translation?
"These are our cooks' knives."
No. Det not only doesn't function as a demonstrative pronoun in this sentence, the demonstrative version of det doesn't translate to this but that.
If it was a demonstrative pronoun it would be "De er våre kniver" and that would translate to "Those are our knives".
Thanks! I'm not sure I understand completely. Can you elaborate on this a bit? What is the function of 'Det' in this case then? Also, is "det" always "that, those" and never "this, these" as a demonstrative pronoun? Even in colloquial speech?
It's just a pronoun that doesn't refer to anything in particular. Knivene hasn't been mentioned earlier, therefore you use the neutral det (used for singular and plural and all grammatical genders).
"Du kan ikke stjele knivene siden de er våre kokkers kniver." = "You can't steal the knives since they are our cooks' knives."
Here you use a pronoun that agrees with the noun it replaces, since it has been mentioned already.
This and these are equivalent to denne(m, f)/dette(n) and disse(pl).
EDIT: Simply put: Det is used to introduce nouns, whether they are mascunline, feminine or neuter, and whether they are singular or plural.