I think a lot of posters are minimising the impact of his acute emotional neglect of OP. His disengagement just at the time OP was at her lowest was cruel and inhumane.
What sort of person leaves someone in the first pain of new bereavement to cry alone?
Even complete strangers often empathize and are very kind in those circumstances.
He's not a good dad at all. His indifference and cruelty are a terrible example to His children. If he were a normally compassionate person he would have offered some kindness and support to you.
Such emotional starvation will inevitably leave you vulnerable to the first person who shows you some interest and kindness. And it was just a kiss FFS!
That's just an excuse for what he has already done some time ago by the sound of it - that is checked out of the relationship. Which he didn't and still doesn't want to admit he's done because that makes him a bad guy and bad dad.
You have had a very hard time OP and are still grieving. You have also been badly let down by the one person you thought you could rely on. That must have been shocking and will have redoubled the trauma of your grief.
You crave returning back to a time you felt safe and at home, but I think that time was a mirage because when that safety and security was put to the test it failed.
Now you need time to recover and gain your strength back. Your DC will be a great source of comfort and strength. Focus on those that are there for you.
Build your own sense of safety and home for yourself and your children. It's within your control and capacity to do this for yourself. You don't need to rely on him again and expose yourself to his inadequate emotional response.