As a DM, there is no way you should avoid subject matter because of the simple possibility of offense to a player... UNLESS you KNOW the situation beforehand. Then it is common sense and good cricket to avoid a sensitive subject.
Many of the games I have run contained players I barely knew -- a friend of a friend who plays and heard we were gaming. Heck, that's how you meet people. And, I've run games at conventions where I knew NONE of the players.
Did that stop me from using my own "Book of Vile Darkness" type of content? No. Serge said it well -- you must make evil reprehensible to make defeating it that much more enjoyable. Evil characters do evil things. Mostly, these evil things are despicable. They don't push granny down on the sidewalk. They break her legs, torture her, and push her in front a car.
If a situation arises in a roleplaying game that might resemble something that happened to a player in real life, that is pure coincidence and not the responsibility of the DM to "water down". That would be like saying you can't talk about a suspense movie because you might offend a starnger standing behind you at the mall.
The only way I change my storytelling approach is if I have good, prior reason to. If someone joins my game and tells me after a few sessions that the subject matter is disturbing, I will deal with it. IF a young person enters agame I am running, I'll tone down the "adult" content.
But I won't tone down any storyline beforehand (whatever the subject matter) because it "might" offend someone or might be parallel to a real life experience of one of the players in my game.
I put the responsibility on the person who has suffered the misfortune to decide for himself / herself whether or not they should play in a roleplaying game where lifelike situations can occur and evil things exist. If they can't get past the personal event in order to enjoy a game played in a social setting, they should probably rethink playing in that game until they have dealt with the grief personally.
I know, it sounds hard-a@@ed, and I feel for someone who has a terrible thing like that happen to them, but I also don't believe in openly foisting your grief on those around you.
Your friends and family will take measures to support you that are appropriate, and you will know if you can count on them for support.
If you are a friend in my game I know you and your background, then I know what I should or shouldn't include in a game -- just like you know what you should or shouldn't say to someone. But strangers and general acquaintences who don't know the situation have no reason to behave any differently in front of that person. That carries into running a game.
And Serge, Cass and I had no problem with the undead children. Honestly. I'll just continue let little Maddie roll all the NPC attack rolls on you in future games...
