What I don't suggest is using any kind of replacement. Like, don't tell your kid the deceased person is just sleeping, or went to sleep and didn't wake up, or anything like that. One of my friends made that mistake and her daughter was terrified to go to sleep for weeks afterward.

death isn't something you don't realized happens, not even kids. they think about it at some point. i realized it without anyone telling me, so if you never tell him it's not like he'll be oblivious..

I HIGHLY suggest you do not tell your children that death is like sleeping, as doing so can cause some children to become afraid of going to sleep. 

If one is religious or Buddhist then it comes in context and is not difficult. 

I wouldn't use the phrase "it was too tired to wake up"... I know of at least one kid who would then freak out every time he got tired... I've told my kids about illnesses and injuries that can kill.  Usually explained that the being was so very sick or so very hurt that the doctor's couldn't help - that there were no medicines or other treatments that would help. I explain that sometimes, rather than staying here in pain the being (I keep saying being because we've applied this to peopel and animals) needed to leave us.

But I go on to explain that even though who ever it is isn't here in their body that they are still in our hearts.  My older son's pre-k teacher gave me that one and it works exceptionally well, even for things like daddy going on a trip or leaving a beloved teacher to go on to kindergarten (we used to "feed" daddy via P's heart).  I explain that some people believe in a place called heaven and we discuss what that might be like. I also explain that some people believe that our spirits rejoin the energy of the world so it can be resused - this very quickly goes on to a discussion of recycling souls like aluminum cans...   Personally, I offer a number of options that people believe and the kids pick their favorite.  The discussion alone tends to take the edge off the grief.

The big thing is to be truthful that the dead won't be coming back, that it's not the same as sleeping, that the cold they caught last week isn't so bad it will kill them and the scrape on their knee won't either.  Kids need to know that it is OK to be sad but that it is important to remember the happy things too.  No matter your religious views, those we love do live on in our memories and kids seem to find comfort in that so this is where I concentrate my energies.  So far, my kids are very matter-o-fact about death in all of it's forms.  They accept it as a part of life and while they may be sad and they certainly don't want death to come to anyone, they can handle it fairly well when I give them room to experience their emotions and then move on.

When I was four my family had to explain the death of my father to me, they used the recent death of my cat to explain it. We're Christian, so it was easy for them to tell me stuff like "He went away to be with God and he can't come back to earth" It was hard to handle, Forever was the hardest part for me to grasp, I thought maybe in a long long time he'd be back. 

This won't be helpful, but my BIL was shot and killed last year.  When I told my 3yo son that my grandfather died, he naturally asked, "Who shot him?"  Obviously Grandpa was old and died of old age.  I guess being shot was his only experience with death.

On another subject, I suggest faith shopping, if you are a believer, or want to be.  Different faiths approach death (and of course other beliefs) from different angles, and you might get some questions answered; something might make sense to you.

You never ever tell a child that death is like sleeping.  That will simply instill a huge fear of sleep.
My sister passed away in January of '09.  It wasn't my children's first experience with death, however, it was their first VERY CLOSE experience.  My son, who will be 11 in May, has a better grasp of what death is than his little sister, who will be 8 in June.
My daughter's grief slowly came out within a few months' time.  She hated going to school, thought that I would die while she was away at school.
When she asked me why her aunt couldn't come back, I have to admit, I had a rough time answering. 
Being Christian (Catholic), we talked about Heaven, that my sister was with God.  I tell my kids that she is still in their heart, always will be.

@ManoAngeliukai9902@xanga - He had an untreated mental illness, while it was a shock to the family, it was not a surprise.  He is in a better place now.  Thanks for the concern.

@Wildflowersp@xanga - Yes, you're right, he is in a better place.  So tragically, though.  Sounds like your family has come to peace with it. 
My sister suffered multiple strokes back in '04.  My parents were her caregivers.  Then Dad was dx'd with colon cancer in '08, right after my sister went into congestive heart failure.  I was a wreck.  My sister passed away in Jan '09, and Dad went into remission just before Easter '09. 
What a few years it has been!