We were giving a stray's newborn kittens a bath to get rid of the fleas and noticed what was left of one of the kitten's umbilical cord was bleeding a little bit where it's attached to the kitty. What should we do?
I don't know, but I don't think you should be giving 2 days old kittens a bath... You said to get rid of the fleas - I am assuming you did not use a flea medicine, right?
I don't know about kittens, but I can tell you something about human babies. When the cord starts to detach there is often some oozing around the edges. It's part of the normal process. Just keep it as clean and dry as you can. The cord will probably fall off in the next day or two and the kitten will be fine.
I used iodine tincture that I got from the pharmacy to put on one of my kittens' cords, the day after I did that it fell off. I would try that, should sterilize the area so infection cannot start or spread.