Welcome to the forum.
1) My preference is for "autistic person", and only when the autism needs specific mention. This is something that gets debated here quite a bit (the words are a bit too common here to make the threads easy to find). It's important to note that there's no unanimous consensus; there's quite a range of preferences among autistic people and their parent or carers. So whatever you decide to use on your website etc., be prepared to accommodate personal preferences when face-to-face. The general trend seems to be a dislike for "person first" language, though the reason why varies; "autistic person" rather than "person with autism". This certainly isn't universally the case, but several prominent autism organisations (such as the UK's National Autistic Society) have dropped person-first language following feedback from service users. Terms like "an autistic" (without the "person") or "aspie" are preferred by significant minorities, but thoroughly disliked by others, so my hunch is that they're generally considered less neutral than "autistic person".
2) I'm not a parent myself (though I was once an autistic child!), but there is one piece of advice that comes to mind. I would encourage parents to use resources like this one to look for mutual support, not only from other parents, but also from autistic adults who's memories can shed a little light on things from an autistic child's perspective, particularly where the child is a bit older or a teenager. Professionals like yourself have a important part to play, but sometimes advice from someone who has been there before you is invaluable.
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When you are fighting an invisible monster, first throw a bucket of paint over it.