It's heartbreaking, isn't it? I think you have to accept that they have a right to be angry about it, it is unfair, and from time to time they may need to express that. The only important thing is that the feeling of injustice doesn't take over their lives.
Another thing is, they may well feel resentment towards a non-disabled parent or sibling. Again, I think, you have to take it on the chin. It is unfair that I, at 45, am far fitter than my 12yo dd and that even her 77yo granny can do far more than she can or will ever be able to.
I do find it hard, but at the same time I am reassured to see that as dd is growing up, she is not actually spending most of her time mulling over the injustice of life. Most of the time she is having fun, discovering new books or music, socialising with her friends, talking to her family, amking jokes, enjoying herself. It's just that she needs to be allowed to express the other negative side when it bubbles up, which is only very occasionally.
My response is, 'yes, I know, it is hard, I feel the same about you'. But I try to say it calmly, so as not to add my emotions. Very occasionally I have reminded her gently that other people have a hard time too (her best friend recently lost her mum), but I take care not to use that argument to shut her up in order to make things less painful for me iyswim.
Ds is much more secretive about his feelings, so he doesn't really say how he feels about just having been diagnosed with the same disorder.
Neither of mine knew they were going to be disabled at the age of 5, but there were other hard things going on- ds's beloved swimming instructor was murdered and he found out . I used the same approach then; let him talk, let him know that there are things in life that even Mummy can't explain, things that everybody has to feel are unjust.