I was 6 when my dad died. I did go to the funeral but not the crem. I had no idea what happened at the crem but I hated not being with my mum and sister - it was as though I wasn't important enough to go. Such a horrible feeling.
If they want to see their father, let them. Let them go to funeral and crem if that is what they want.
Try not to let anyone put down their grief - just because they are young does not mean they are not suffering. I still remember someone telling me after the service they knew which hymn I'd chosen as it was nice and jolly. That was not the reason, and I hated someone making light of the situation.
Let them see you're upset, that it's ok to be upset and if they say something which makes you upset that's fine too. Don't put them into the situation where they feel they have to shut down their emotions to keep you happy.
Talk about him, share stories, look at pictures of him. Maybe make some memory boxes. Write down things he said about them.
Don't be afraid to find someone else when the time is right. My mother didn't, mainly to protect me, but it did me more harm than good.