Depends on the age of the kids
don't lie...but find a softer way of telling them the truth..and make sure they know you love them.
divorced? on the road? deadbeat? working double shifts? It would depend on a few things
Be honest and fair, but not judgemental. When they are old enough, they will figure out why. Is there a valid reason why he is hardly there?
Women are more mature than men at that age or any other! Again, why is he not there for your child? Maybe he will step up to the plate some time and be a man and a father! You can be honest and gentle and loving and let her know that you will always be there to love her! I hope you have a good support system in the way of family and friends.
I usually don't say anything unless they ask. When they ask I just tell them "I guess daddy is really busy with work and other stuff, I'm not sure- so when he does call you, ask him." Its a long story.
Not much Don't speak against him remember they will become aware of his 50% in degrees on their own.