Catapult No - I think you're right. If you need a dog to assist you with your life, you should continue with finding one from either Canine Partners or Dogs for the Disabled.
Taking on a puppy is hard work just to have one as a family pet - to train it to fulfil a specialist role is another thing entirely.
The dogs that are used for assistance work, whatever the disability, have very specific temperaments and a fabulous apptitude for learning; and even when they've been sourced and trained by an expert they still sometimes don't make the grade. It would be all to easy to get a dog yourself and find you're in that position.
And you'd not have the back up that the two organisations provide, either. Just in case something goes wrong. I do know of someone's dog who became too possessive of her about a year into their partnership. I can absolutely understand why it happened, but he couldn't stay with her. It wouldn't have been fair on either of them. He's since been retired and has a happy life with a member of her not much extended family.
The umbrella organisation which might be a good place to start is www.assistancedogs.org.uk/
Sydney I don't know much about assistance dogs for children, but I do know of a number of children with them. The majority of assistance dog organisations do offer dogs to children over 7, I think. And I think the only major one that doesn't is Canine Partners. The umbrella organisation above might well be a good place to start.