Cahms usually start by building a case history/background and to do this they will get information from different folk to get an all round perspective. Once they have gathered the information they will decide on the best way to help. This could be working directly with the young person or working with the family to help them develop strategies and support the young person or work with the young person and family together or a combination of these.
If they "get" ASD then it should prove helpful.
Don't feel threatened or out of your depth. They might be experts in mental health and behaviour, but you as a family are experts in your children/step children. Without your expert knowledge and insight, Cahms wouldn't even be able to make a start.
(ex-foster) Mum and single carer to S, who is 21. Has ASD, epilepsy, IBS and displays challenging behaviour when anxious or hormonal.