Ok, we got to back up way up here--
First @LewisC, your children are still mourning the loss of their mother -- which is a completely different feeling that you mourning the loss of your wife. Be there for them. Build that relationship. Visibly honor their mother, and their mourning. You got to be there for them. Listen to each one individually. Don't ever try to forcefully speed up any one of their grieving process and working through things. This comes first and foremost.
Next, there is this idea of each of them welcoming another adult woman into thier temporal lives (not even talking about sealing yet). This woman will never ever be their mother. But the idea of you marrying her is... obviously unsettling. Yes, you can and have found another woman you love. But they will never have another mother. So this obviously brings all of those feelings of mourning to the surface -- feelings that a mature adult would obviously struggle to work through, let alone a kid whom loss his mom at age 6. This needs to be taken slow and respectfully.
Loss of a parent is a gut wrenching trauma, especially at a young age. No rational conversation is going to make that feeling go away.
