The kittens should be fed high quality canned cat food. You can find these
in pet stores. They should also be seen by a vet and provide stool samples
for each one. Most kittens have worms and this will cause gas and
diarrhea.Do not give them milk.
Gail

Make sure the kittens are getting kitten food. It's a lot higher in fat
and protein which kittens need more of since they are growing fast.
Good brands I've liked are Nutro or Royal Canin but those can be
pricey...Iams, Science Diet or even Purina One should be ok if you
can't spend so much.

If they are just getting the gas and diarrhea recently I don't think
it's their mom's milk. Take them to the vet if changing the food
doesn't help.