I definitely am not making anything sound like an ultimatum at all. In fact this is the type of thing I would say
"H, I am struggling with trying to say this in a way that you understand that I am coming from a point of concern for the children, and not trying to influence your decision to serve my own needs. Please believe I am sincere when I am saying that this situation (fill in the blank) makes it difficult for the boys because of (fill in the blank). It might be easier if you find a better way to go about doing this and here are some examples of what might go wrong, which I hope to avoid. Are there any considerations you might want to talk about before you think about a decision? Okay, then I hope to hear from you later to hear your thoughts."
I used to write emails like this too. Never really saying what I needed and expecting a result. I learned here and from my IC and some other resources to ask for what is needed.
H, when you are unable to visit the boys, they tell me they're sad; they want to have a R with you. Would you be willing to give set times when you can visit and if you can't make it call and let us know.
Have you read How to Improve Your Marriage Without Talking About It? It really opened my eyes to dynamics that had been hurting my M for years. Since your H sounds very like mine, and you and I probably have similar characteristics, you might appreciate it.
Me 57/H 58 M36 S 2.5yrs R 12/13
Let me give up the need to know why things happen as they do. I will never know and constant wondering is constant suffering. Caroline Myss