ExH stopped contact in a very brutal way, DS and him were very close before and after the split (we split when he was four) , but his second partner is a very controlling and aggressive person who didn't like DS much. DS's dad is the kind of person who will chicken away from confrontation so he just kept quiet or went to another room, when his partner was verbally abusing his 6 year old son,
Over the years, I have tried to raise DS without hate. Don't get me wrong, exh was such a bastard with DS that my heart is full of hate, but hate ruins your life so I have tried not to let DS go in the same route as me. The worst thing you can do is hurting your children by getting their heads full of resentment towards their dad.
You don't need to lie, they don't need to know that his dad "took a bad decision" or that "he doesn't love his mum anymore". That is terrifying for children so young.
Tell them the truth, in an age appropriate way, they don't need to know how bad things were, it is enough for them to know very little so you can give them other parts of the story, as they grow up and when they are ready to digest it.
What worked for DS at that age was:
Q: Where is dad/when am I seing dad?
A: I really wish that I could tell you, but I am afraid I don't know myself (let them talk for a bit, then distract them with something else, park/Ice cream will do)
Q: why is he gone?
Mum and dad were not getting along very well, and as you have seen, we were not very nice to each other so we thought it was better for dad to move out.
Kids feeling miserable? Just ask them "how do you feel, and let them talk, don't ask questions but answers theirs kindly (see above) and when the conversation is dying down, propose to do something nice.
Before he stopped contact, exH was very unreliable with contact, so I prefered NOT to tell DS his dad was coming, to avoid him feeling miserable if he didn't show up.
The big truth is that being abandoned by dad is a huge tragedy, and they will find it difficult to understand why his dad acted that way. Your role is not to inform them their dad was a bastard (even when you, the world, and their dog knows he IS). Your role is to undo the damage he has caused as much as you can, so they can grow up as happy as they can and in time, develop healthy relationships.
I wish you all the best in this difficult route. It has taken me years to accept it, but I know now that exh disappearing from DS's life was actually a blessing in disguise. I cannot imagine how much more damage he could have inflicted on DS if he had stayed in contact.