Since your cat has fleas that means they are in your home. The one and only thing I suggest a flea collar for is to put it in your vacuum bag/cannister and then any fleas you suck up when you vacuum are killed.
This is good advice, but wanted to add that it's too dangerous for newborns. OP is right to get this under control before the kittens are born.If you need to de-flea kittens, the best way is to bath them, using aDawn dish soap or I have used Johnson & Johnson unscented baby shampoo. Check for detailed instructions on bathing kittens on this site.