C.S.
Dear L.,
First, if you haven't already, don't tell your children they are going to see daddy. Just tell them you are taking a road trip. Then if he does not show, they won't be disappointed.
Laurie is absolutely right. I really don't see the harm in taking them to see him for dinner or even a round of mini-golf, or bowling. Your 4 year old may not remember him if he hasn't seen them in 9 months.
I also know that this is probably not your choice of a fun evening but if the father wants some contact - it is a relatively safe way to accomplish that. Your boys did not chose their father - you did - and they did not chose a divorce - you did. Try to remember that children love their parents despite their parents failings and try to respect that while protecting your children.
I agree that you do not want to leave them with him unsupervised after they have been admittedly abused in his care.
Be safe.