Hmmm...actually, that's not such a simple question. It's thought that there are both dominant and recessive forms of polydactylism. Offhand I'm not sure of the answer but I'll let you eavesdrop while I work it out.
Okay, we know that if the gene is dominant, Mom is heterozygous - meaning she has an unmatched pair of genes, Pp. We know this because she has non-polydactyl kittens. But if the gene is recessive, Mom is homozygous, meaning her genes match - pp. We would know this because she is polydactyl.
We can't know for sure whether the ratio of polydactyl:non-polydactyl in the litter is actually the ratio that would be statistically expected, but since we can't actually test that we'll just have to assume that the expected ratio would be 50-50.
So for different parental genes, this is what we'd have:
If the polydactyl trait is recessive:
Mom - pp, Dad - pp - all kittens polydactyl. We know this isn't right because there are non-polys in the litter.
Mom - pp, Dad - Pp - 50% polydactyl, 50% not polydactyl. This is very possible.
Mom - pp, Dad - PP - no polydactyl kittens. This isn't possible.
If the gene is dominant:
Mom - Pp, Dad - Pp - 75% polydactyl, 25% non. Possible but not likely.
Mom - Pp, Dad - PP - 100% polydactyl. Not possible.
Mom - Pp, Dad - pp - 50% polydactyl. Very possible.
Hmmm, so it looks like after all that I still don't have an answer for you! If the gene is dominant, then the kittens who are not polydactyl would not carry the gene. If it is recessive, then all the non-polydactyl kittens would carry the polydactyl gene.
The only real way to tell would be to see if you ever had polydactyl kittens from a non-polydactyl mom. I can't in good conscience suggest that anything other than spay/neuter would be a good idea for barn cats, of course, but it would be the way to get the answer.