Introductions take weeks and months sometimes. Keep the kittens in one room by themselves with their own litter pan, food/water. In a few days reverse the cats - put the adults in the kittens' room and let them out into the house supervised. This way they can check each other's scents without face to face meeting.
Then sprinkle all the cats with cornstarch baby powder so they smell the same. Put the two kittens in a carrier and let the adults see and smell them.
Expect some hissing, but let the kittens out and supervise. Don't interfer unless someone really is in danger. They will hiss, growl and maybe swat at each other . Be sure that ALL nails are clipped short on all the cats before you let them together face to face.
We had Charlie confined to the library room for a few weeks and it took Ling several months to accept him. She still doesn't accept him totally cause they get into arguments a lot
Females tend to take longer to accept new kittens/cats then males. I'm assuming the adults are altered and the kittens will be done shortly. They can and do bred at 4 months old.