Oh my goodness... This is totally heartbreaking for you and your kids...
My kids are similar in age to yours and my xAH lives in the same town and is rarely involved.
My just turned 6 year old daughter is very sad about her dad emotionally abandoning her but sadly he did that long long long before he left...
It sounds to me like you're doing the best you can in the circumstances. Telling the kids everyone loves them in the family including Mommy is the right thing to do just as you have...
I know I often wish I could shield my kids from feeling the hurt caused by their A dad's absence in their life. But I have slowly come to accept that they are going to be sad and grieve just like I had to over the loss. You sound like a great Dad and thank goodness you're there for your kids.
My heart hurt for you just reading this and I got a lump in my throat imagining the scene and your little one asking about Mommy...
So sorry... Thinking of you....