Depends on the Dog and the child.. If you have an "assistance" dog, that is trained to notify if child is in danger, that may be of LARGE assistance. However if you mean as companion for child, then do you have a dog that is of a friendly, loving breed, it would need to be a tolerant dog, for sometimes autistic children make inappropriate gestures and touches that would irritate say a poodle or a "pretty" dog like a shitzu.
The benefits to the child are enormous, and not just to autistic children. Firstly, if raised around children, most loving pets have the ability to seem to understand their humans, thus giving the child someone that will listen to "WHATEVER" the child has to say, without judgement, comment, or (and this would be most important to children with autism) without correcting what they were saying to an "acceptable" grammar. Secondly, it would give the child a constant companion, that was always there. Thirdly, the love that a pet shares is non-judgemental and usally unconditional.
Unfortunately, a slight problem may occur if the child in one of those that needs STRICT scheduling to maintain normalcy/connection. A pet of any kind "can be" unpredictable. Dogs don't ALWAYS have to go to the bathroom at the same time' don't sit in the same place, etc.
I have spent time with autistic children, some having pets, and some not having pets. The ones WITH pets seemed more open and aware of their surroundings. They did not get as frustrated with changes in routines; wait-times were no longer tragedies, but excuses to pet the puppy longer.
I hope this Helps!