I believe it is never too late to grieve. In fact, if you don't, your sadness over the loss of your children's father may be prolonged indefinitely.
Your children will ask about their father someday. And if they see that you haven't grieved fully, they may also have a hard time grieving the loss of this man in their lives and accepting his death. They are relying on you and your feelings as a guide for their own. If you feel ambiguous or have stuffed your feelings deep inside--unable to move forward--that same behavior may be passed on to your kids.
I recommend that you allow yourself the time to grieve (yes, I know it's tough with young children), reflect on your life with your soulmate, and even go so far as asking his advice to help you raise your children from time to time. If you talk to him quietly in your heart, you will always receive the guidance you need.