I'm not sure what you would say to those particular age groups, I spose it depends on how much they understand about your dad dying. This might be a stupid qustion, but have you talked to them about why he died, ie. what illness he had?

We went through similar recently when DH's dad died in February from cancer. Lucy is only 2.7 but she was really clsoe to her Grandad and visited him a few times a week (they lived a few streets away). We didnt want to say anything to confuse her but I didnt want to lie to her either cos she would have to deal with it sooner or later.

So in the end I said something like this:

Me: "You know how Grandad was really sick and in the hospital?"
Lucy: Grandad sick.
Me: "Well his body was very sick and very sore and so that he didnt have to be sore anymore he died. That means he doesnt have to be sore or in pain, but it also means we cant see him anymore and he wont live with Grandma anymore either."
Lucy: Grandad live at Grandmas?
Me: "No honey we cant see him anymore, and today we are going to go say goodbye to Grandad at his funeral.
(She stopped talking about now and just nodded as I spoke)
ME: And we're all going to miss seeing Grandad very much, and it is very sad. You will probably see lots of people crying today cos they will all miss Grandad too, so the best thing you can do is give them lots of cuddles, and if you want to cry too you can ok?
(Lucy cuddled me and then promptly changed the subject).

I would just try to have an honest chat with them about how you are feeling and how much you miss him and are sad, and then they might see that everyone's in the same boat, its not just them, and that you can get through it all together.

Its hard cos everyone grievs differently. My DH is very reserved (comes from a very reserved English family) and doesnt like to show his emotions outside of me & the kids, so its been hard helping him grieve his dad's death as I never know when tis worse for him or not as he keeps it all bottled up.

Big hugs to you
xxxCaz