I have duck eggs hatching and the mother was killed?

Add a source of humidity (ie a shallow dish of water near the ) Other than that you are giving them the only real chance of survival. If they are wild duck eggs if the chicks hatch get them to a wildlife rehabber they are illegal for you to have. If they are domestic provide an appropriate heat source especially at night for the first week or so, an appropriate chick starter feed and a water source small enough they can't get into it (young chicks don't produce their own water proofing oils they rely on their parents for that, so allowing orphaned chicks into water before they produce their own can cause issues)