My mum could have written your post 50 years ago, OP, even down to the ages of the children. I was the 6-year-old in your scenario, so here is my perspective from the child's point of view.
My dad was a stranger to me. Yes, he was "home" at the weekend but always away doing things: the TA, seeing friends, pottering alone in his workshop (if we ventured in he ushered us out).
Every fun thing we did was with my grandmother coming along where he should have been. Even holidays were with DGM, although sometimes my dad would drive us all there and collect us at the end. He never came visiting with us (remember when people did that?) not even on Christmas Day.
When our DM became ill and was in hospital, we went to live with out DGPs for about a month and he didn't visit us once, or offer to give them a break.
He never took us out anywhere, never gave us anything. He didn't engage much with us at all, and was quite strict with us when he did. To me he was this grumpy person who showed up occasionally and it was a relief to me when he went away and we could get back to normal again. Not so my 8-year-old DS who was Daddy's girl and would cry herself to sleep for him, always asking our DM where Daddy was, when was he coming back again.
Turns out Daddy was having a great time, he travelled all around in his job during the week, with a girl in every town, and apparently met his next wife at the dancing.
I have only a handful of memories of him being a part of my childhood, few of them positive. He just wasn't there, even when he was, if you know what I mean.
OP, I know your DH won't be quite as bad as my Dad, but if you do try to talk him about spending more time with the family, it might be an idea to mention that his absence will be having a hugely negative impact on the children too. If he wants a relationship with them in the future, he needs to put the time in now, or he could end up being a stranger to them too.