Most vets use disolvable stitches now when they spay/neuter a cat. They should have given you a hand out of what you can and cannot do. The discharge instructions are different for males and females. If you do not have the paper or do not feel it is clear call your vet and they will tell you if they can bathe or not, make sure you let them know they have fleas and they did not have them before they went in so they can treat the problem at their facility also.
I would not give them a bath at all. Just get a flea comb and go over their fur with it. If they had not had stitches I would have bathed them in dawn.
Also there is a shot you can give them that kills fleas in 24 hours you can get from the vet.
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