I am in your shoes. A relationship of nearly 5 years ended this year. He too had younger children. (Similar in age to your exes).
One of the things we had always said when dating was that we wouldn’t just disappear from our respective kids lives. I didn’t want any of the children (mine or his) to suffer further loss having already come from divorced families.
The reality some months on is that my children haven’t mentioned him once. They have not asked after him or asked to see him (that being said, they are older and they’re dad is very much present in their lives so it’s not like a critical male figure has stepped away).
On the flip, I feel that he has used his children in order to stay in contact with me. The children have my number and have on/ off texted/ facetimed me. The last weekend I did it, they asked when they were seeing me and my children next. After that call I realised he had not told them we had broken up. I personally do not want to see them as previously that level of contact has eventually lead to reconciliation. Which I am not open to. I also find that time really hard because hearing little children I care about saying they miss me was awful and it set me back.
The reality is that if children are this young and have their key parents in their lives, they will adapt and move on very quickly. I personally think it is not your responsibility to tell them the relationship has ended. It’s his. He should let their mother also know so that she can also reassure and comfort the children.
After he’s told them I think its an option to say goodbye, I do think that is healthy. And whilst it might be very sad, it will give children closure. But you only do that if you feel you can do it.
Ongoing contact? My personal experience says no. It’s too painful. It keeps lines of communication open. And it stops the healing process / stops you from moving on. He also will at some point meet someone, how will that work?
I have told my ex to tell his children that he and I are no longer together. I have also asked that they no longer contact me. Ties need to be cut and we all need to move on. If the children were older and old enough to independently retain a relationship with me, I would have done so.
This young? It’s too painful and it’s not separate from the ex. That’s my experience anyhows.
It does get easier x