I worked with quite a few kids who have parents with mental illness so here are a few suggestions from my experience that you would need to tailor to the specific situation...
I completely agree with telling. Kids are intuitive and get a feel that something isn't right. Some of how you tell is going to depend on the cause of the depression and PTSD. If it is war or a car crash or something the child is aware of then that is different from sexual abuse or something the child doesn't know about.
Keep it simple...at 6, no matter what you say the focus should be on making sure the kids...
- - Feel safe,
- - Know that is isn't their fault, they didn't cause it
- - Know its okay to be sad / worried that dad is sick, and want to help (and can help) but that they can't fix it - can't cheer dad up. Up to adults and doctors to try and fix it.
- - Know that dad still loves them the same
- - Know that dad is working with doctors to get better and this isn't forever.
- - Feel comfortable to ask questions
I would give it a name - either depression or PTSD but not both. Go with whatever one is most prominent. Ask if they have noticed if dad is any different, what is daddy like at home...then let them know that sometimes daddy gets sad/mad/needs to be alone because he has something called depression. Or sometimes dad has nightmares during the day / night when he remembers something scary (tell the event if appropriate) because he has something called PTSD.
You can say something like "I know you worry sometimes because I don't feel good. Sometimes I get cranky, or get really quiet, and I can't help it. I know that this can worry you. I don't want you to be worried about me. I'm OK, and I'm getting better, and I'm going to take care of you no matter what happens. Even if I'm not feeling good, I'm still your dad and I love you and I want you ask me any questions you have anytime."
You can compare it to an illness the kids know...even a cold, how other people can't make it better, even dad can't help it or make it better by himself and that dad will have to check in with the doctor / take medication to make sure he is getting better. I wouldn't go into detail about the symptoms of depression / PTSD. Instead focus on what the presenting behaviour is...to get better daddy is getting help to sleep better, to get more energy, to get help calming down, to help with his nightmares, to stop crying so much, to not want to be by himself as much etc...
Talk about how hard it is to see someone different and that's its okay to miss the things they used to do with daddy when he was well. Also really important for dad to keep as much of depression / PTSD from kids as possible. Kids an be very irritating when you are struggling with mental health as they are loud, active, etc... so it is important to have times when dad has quiet..maybe someone takes kids to the park.
Let kids lead conversation to some degree. Don't give more detail than they want to know beyond the basics. Check in periodically with them to see how they are finding spending time with dad and if they have questions.
If it is war related there is a book that is available online called Daddy's Home - http://www.carolinanadel.com/PTSD/