If it's an option for any of your children, Dragon Naturally Speaking (voice to text) has utterly revolutionised my life! The latest version doesn't need to be trained to your voice in the way that previous versions did, and learns the things that you say most often, which really improves accuracy. It's obviously not appropriate for classroom work, but for homework, and particularly essays, it's brilliant (less good for maths/science unless you're prepared to put in a lot of work training it).
We got Dragon, hoping that it would help my son. But he found that he froze when expected to speak, didn't know what to say and so on. But then he is like that if people talk to him. He can answer questions directed to other people, but can't answer questions he is asked directly.
However, I found Dragon a life saver as just after we got it I did my wrist in again (how did I get to my 40s without realising that it was the hypermobility causing all the problems with my wrist?) and it was great to be able to rest my wrist and still communicate with others.
So yes, I think Dragon would be great for older children and adults who have problems with writing, but doesn't work so well for those kids that also have expressive language problems.
And I guess that is one of the difficulties for some. They don't have just one straight forward difficulty, but several things going on at once that all interact with each other.