*hugs*
I know hospitals have all kinds of different rules and stuff, depending what part of the hospital they are in...but I'm wondering if maybe a local friend or family member can smuggle in a smartphone to him, so you can have a direct private line to him, and he to you, that either of you can use any time, day or night. Or even use Skype, perhaps; or set up a private tumbler blog where you can take a picture of yourself holding a sign with a message every day--and maybe encourage him to do the same for you. You might even be able to get a particular nurse to help. Send him a word or number puzzle every day--tear a page or two out of a big book of puzzles, put them in an envelope with a personal note that you can pop into the post every day, so he can have something to look forward to. Share a chapter of a book once a week, and your thoughts, etc. ask him to pick something, like when you were little, and then you (possibly and your kids) will send drawings back of that subject.
Those are just off the cuff ideas--the point of them is to make you feel involved from the prohibitive distance, and for him to know that--no matter how much pain he may be in--you are with him. It will also give him something to eagerly look forward to, as opposed to dwelling on his illness. Obviously, you know your family and how to adapt such things.
Please keep talking to the therapist, and try to take every chance to redirect your anger and frustration towards being away from him and your kids right now into some way to show him how important he is and how much you all love him. God forbid, he does poorly through the rest of his chemo, you will hate yourself down the road if you surrounded him with discord in his last days than adventure, fun, affection, adoration, etc.
I mentioned before; I was overseas when my dad passed from cancer. I had the chance to call him a mere two days before he died, and I let myself be pushed into something else, and made to feel guilty about the cost and timing of any calls I made to him. I don't think I've forgiven myself for it, simply because I don't know what state of mind he was in. I know it happened suddenly; he was being taken to his next chemo treatment, just like any other day, and along the car drive, he said something felt different and asked for the car to be pulled over, and then he slipped away in the ambulance.
Make sure there are no remaining differences between you; the physical distance that separates you doesn't have to separate you in he ways that really matter. Keep yourself open, and give the therapist a real sold chance so that you can give your father and your kid the support and love they need.
And hell, we are here for anything you need. Feel free to PM me if you want.