In your opinion, if a child has severe autism, is it a better idea for them to see a music therapist or will they get the same thing out of regular music lessons from teachers not trained to work with people with disabilities?

Can't answer that one without more info. How old is the child? How do they respond to you? What do the parents say? Is music their thing? Music lessons, even if you are new to this area, could really help them socialise. There is a lot of info out there on the web. Can the parents give you contact with child's Senco (if in mainstream education)? Have had contact with severely autisitic children in a comprehensive, and with a little forethought, you would be surprised what can be achieved.

Sorry, have just realised that you are from Canada. A senco is a special needs co-ordinator who is also a qualified teacher who assesses and organises support for special needs children in mainstream education. Autism, even if severe, can be improved/socialised when the sufferer undertakes a programme of lessons in a subject they can enjoy. It takes a lot of thought and preparation on your part, but individual lessons for a instrument could be ideal. Be guided by the parents/carers and any educators they have contact with.