My cat had two kittens 24 hours ago and they are not feeding from mum yet... how do I latch a kitten on to the nipple

Try getting them to open their mouths, and then putting them to a nipple. If that doesn't work, then you HAVE to talk to a vet about tube feeding options. It's already dangerous at this point that they haven't been eating. Kittens, especially newborns, can starve to death VERY quickly. Even waiting another day, may be too late to save them.