You have TWO witnesses that say it was the Dad doing the pushing. Your DD would like to be in contact with her dad again, so it's very unlikely that she would lie to support her brother.
You have an adult who threw his children out on the street to wait for you to collect them, you witnessed that part for yourself. I'm sure that didn't take place without some very unpleasant comments, which must have been humiliating and traumatic for them.
Why is his side of the story given so much credence, because he's an adult?
"He has always been focused on respect and obedience from the kids"
and yet he makes no attempt whatsoever to set them an example of how to treat people with respect.
I'd be apprehensive about sending them back, because next time he gets mad with them - how is he going to top his current display?
Any adult who can cold shoulder children for six weeks until they get the apology they demand, but who will not consider apologising for their own behaviour in turn... or meeting them halfway, does not deserve any apology at all.
Imagine being a child, not knowing if they will hear from their father that he doesn't want them to visit, imagine the uncertainty of this lasting week after week, not knowing how long this would go on for or if that's it.
Its dreadful to think of that. They must feel he's disowned them. As in fact he has.
And if he eventually gets the much demanded "apology"... what then?
Will that be good enough?
He's made such a stand by now that I doubt a simple apology will satisfy him and I very much doubt from the sound of it that he would acknowledge that he was at fault in this scenario in anyway.
He's a cruel man. That's all you need to know.
Let him come to his senses.
Claim full time child maintenance.. that might sway him.