Libby, my kids were 15 and 13 when H left. He had very little to do with them and was impatient and short with them when he did see them. It was as if he was dropping his responsibilities as a Dad. He would make arrangements to visit them but would cancel or change the plans to suit himself. He didn't attend their school/sporting events. As time progressed he had a regular dinner night (at my suggestion) and would occasionally invite them for another activity but the invite was usually at the very last minute. (It was as if he would see how he felt and if he could cope with them)Often these activities were quite tense.
The kids learnt he wasn't reliable and had no expectations of him. They stopped contacting him, refused some of his invites and didn't bother with gifts. They often told him what they thought of him and various situations. They learnt to speak out and not just accept his cr$p.
I got involved at the beginning, trying to be the peacemaker and to right the wrongs. It was only when I stepped away from it all that H recognised that it was lonely without the kids and started to make moves to restore his relationship with them.
Now, 2 years on, they quite often go there for meals or just to visit and things seem to be much calmer and happier. He's back to being a Dad.