"Because of another prophecy," said Mr Beaver. "Down at Cair Paravel - that's the castle on the sea coast down at the mouth of this river which ought to be the capital of the whole country if all was as it should be - down at Cair Paravel there are four thrones and it's a saying in Narnia time out of mind that when two Sons of Adam and two Daughters of Eve sit in those four thrones, then it will be the end not only of the White Witch's reign but of her life, and that is why we had to be so cautious as we came along, for if she knew about you four, your lives wouldn't be worth a shake of my whiskers!"
This is an invented idiom by C.S.Lewis. It's an idiom he made up, that applies similar structure to actual idioms , but one that incorporates the fact that the speaker is a Beaver and has whiskers.
not worth a whistle
Worthless or useless to such an extent that the person or thing in question does not merit the effort of being whistled at.
I was so excited when my grandpa said he'd give me his car, but this old clunker isn't worth a whistle.
Since "a shake of [someone's] whiskers" is presumed to worth very little, similar to the worth of "a wrinkle of [someone's] nose" potentially for a human version.
The effect of this invented idiom is to say that the four children's lives would basically be worth nothing. The queen would have no problem having the children killed.