In that case my own dog came via lurcher link - can't praise their matching skills enough. We took 6 months to find & choose our whippet via them and she really is the best therapy my AS son has had. Animal Samiritans and some of the specific breed rescues take similar time and care.
Or go for an assistance dog failure - the most common reason for a guide dog to fail training is lack of confidence (esp in London!). Dogs like this make wonderful buddy dogs for our kids.
Write a list of what you can't handle as a family, but do choose a young dog if you don't want a puppy.
e.g Do not want to have to do 2x 5 mile walks per day in bad weather, don't want huge poops to scoop, mustn't be too boistrous, must be able to cope with living in a flat.
An eliimination list is suprisingly helpful for breed selection as you want a dog that fits your lifestyle given our Kids are so time consuming anyway.
Gundog breeds like labradors and golden retrievers are popular choices.
Cavalier King Charles spaniels are nice for children that need a smaller dog with a gentle temperment.
Greyhounds/whippets are great for homes that want a couch potato and tactile children
A jack russell is like a duracell battery and is very protective but cannot handle a child that might be a bit rough (they can nip).
I would absolutely avoid staffs or any of the breeds that have a "rep" as you want the dog to help you strike up conversations with strange children and old ladies in the park and not cross the street to avoid you. (There is nothing wrong with staffs btw but they have the wrong public image for an Autistic child to reap the full social skills benefit of dog ownership and this is a critical desired benefit). Similarly avoid breeds with a very strong guarding instinct as you don't want a dog that leaps to defend your child from his playmates if he has a melt down.