Have a look at Winstons Wish website - they had good advice about telling kids and helping kids cope with bereavement.
Have a rough idea now of what you think you'd like to do about the kids and the funeral. But leave it open so that you can change your mind when the time comes. The kids may want to come, or they may let you know that they do not want to come - quite apart from what you think yourself.
One of my gran's recently died (not cancer but we knew it was a matter of time) and DD (6) got to see her in the hospital a good bit over the final few months. She didn't come into the room the afternoon before gran died (she was unconcious at that stage) but had had a lovely visit 9 days before and a great chat together.
I talked to her a lot about death in a factual way, and also what to expect (grownups being sad and crying, or getting stressed and maybe being a bit cross, or perhaps just wanting lots of hugs) both emotionally and talking through the practicalities of what would happen and when, and that she could be sad if she needed but that it was good to be happy too for gran. DD is very literal anyway, but she was very matter of fact about stuff, which helped.
When my grandad had died 2 years ago, we brought her to the removal (evening service) but not the open coffin in the house or to the funeral itself. She has been to visit the grave a few times though (and also my other grandad who died before she was born - recently deceased gran's DH). This time, I had to bring her to the house when the coffin would be there, but I left it much later to arrive myself and told her all about it but that she didn't have to go into the room if she didn't want to (I was happy to bring her in if she did - but she chose to stay out). There were plenty of family around and "helping jobs" (like bringing around buns, or clearing dishes) that distracted anyway. Again though, she went to the creche for the day of the funeral (they were running summer camp for schoolkids and had space to take her).
Over the past year, as both gran's have been ill and fading slowly, I have given her the option of coming with me to visit or not, at home or in hospital. With (just died) gran, she wanted to visit a lot and we did. With other gran (93, bedbound and suffers from dementia but happy in herself just no short term memory and now forgetting who we all are), she has always found her overpowering anyway (and so do I, TBH), so she was less inclined to visit. I have been able to visit without her and not have any issues - and she has asked to come occasionally now that gran is in bed all the time, to see my aunt living there still mostly, but sometimes pops up to say a quick "Hi" to Gran too.
If you talk to them in an age-appropriate way, let them know they can ask you anything, and give them the option to visit as much as you can (either it may not be feasible for them to come, or for you not to bring them - but they don't have to go in to see your Dad if they would prefer not to, bring some colouring in or a computer game or book for the dayroom or the car), it will all help. Life and death are all natural, and kids can be remarkably matter-of-fact about it all and accepting of new realities.
Mind yourself too.