The dc weren't 'found by a neighbour', Camel.
After their df abandoned them on their doorstep and drove off knowing that it would take some time before the OP could get home, the 7 yo dc told her 4 yo sibling to stay by their front door, which is almost on the street, while she crossed the road alone and went to the OP's friend's house 100 yards away.
The entrance door to the friend's house is set on the side and was not visible to the 4 yo who would have watched her dsis disappear out of sight and, at her young age, it would have seemed hours instead of the minutes it took before the OP's friend came to scoop her up and take her to her home.
In abandoning his dc in this callous manner with no regard for their wellbeing or safety, he's committed a criminal act which is compounded by the fact that he did not return within a short time to check on their welfare/make sure the OP was home, nor did he make any enquiry after them until some 24 hours had elapsed when he sent the OP an email asking when he could see "my girls" again.
The OP has not said what effect their df's behaviour has had on her dc but it wouldn't be surprising if the needless trauma they sustained is such that they may now be exceedingly wary of being left alone,with him.
As literally anything could have happened to his dc during the time they were left alone on the street or, for that matter, to the OP as she was making her way home to them, 'outrageous' doesn't even begin to describe what he's done.
His behaviour beggars belief and it's to be hoped the police decide to throw the book at him.