I'm sorry @seethebeauty84 that's an awful thing to have to deal with.
Winstons Wish is a bereavement charity for children, they have fantastic resources to help explain the loss in child friendly ways. I lost my mum when I was 21, my brothers were 18, 16 and 5. Then dad died when I was 28, my youngest brother was only 12.
You'll probably find with your children that they'll have periods where they seem completely fine, then all of a sudden it will hit them, then quickly they'll be fine again. It's how kids process grief, unlike adults who will feel the deep pain for a prolonged period.
Take any and all help you can get from family and friends, let them take the kids out for fun times if they are able to, look at kids clubs and activities that give them normalcy and you some room to breathe.
Get your paperwork in order now, probate, wills, if your husband has bank accounts in his name, get added so that they are joint accounts and don't get frozen, look at your DHs employment contract to see if there are death in service benefits, also if you have life insurance check for critical illness cover.
In the weeks and months after you pr husband passes there are no 'shouldn't when it comes to your feelings, your kids feelings, house keeping, diet, exercise or any of that stuff. If you eat nothing but fish and chips for a month it will be fine. Do not put any pressure on yourself.
You also might find that you feel unexpected emotions, I remember feeling so ashamed of myself because I was just so fucking angry at my mum for dying and leaving us. If you have someone you can be honest with then talk to them, if not then look into counselling because it is useful to be able to vent.