I am sorry you are feeling so worried, Firsttimer. This post is going to take ages to write, because i will have to do it in dribs and drabs whilst 'managing' the chaos of 3 dcs each trying to destroy the house/each other/my sanity - so bear with me!
I just wonder what support you have now? It varies so much from area to area, but I'll say what we have and see if there is anything you feel you are missing -
ds has had a SOGS and a ruth griffith assessment, the ruth griffiths is far more accurate at assessing the level and areas of delay.
he sees a physio and an ot, every couple of months, to discuss ways we can help him physically to play, have his Piedro boots fitted, discuss what equipment might make our live easier.
he attends a special needs nursery, twice a week - i go with him, and he sees a SALT here (although I'm not happy with the way this works), they have key workers who carry out a programme designed to help each child, which is worked out by the physio and SALT there, they also have soft play, senory room and music therapy. It is a free nursery which works on referral basis, but ds also has a CAF statement which means funding is available for him, i believe this is assessed regularly and will change as he heads towards preschool/school.
he sees his paed once every 2 months, this is to discuss various medical issues and arrange tests etc
we are just about to start weekly portage
he has an annual team around the child (TAC) meeting, in which all professionals involved in his care, and us, sit around a table and discuss the best support for him, and what tests etc are needed. He had one at 10 months and is due one in May.
We have a very very supportive and helpful gp, who we see every couple of weeks, she has been our strongest area of support really, always helps contact people, and makes sure I am not overwhelmed (I saw her when ds was first assessed suffering with a lot of panic attacks and anxiety).
Some of this works well, other stuff I feel needs to be changed, and it does seem sometimes that I spend my whole day trying to arrange meetings appointments, etc.
I find the really exhausting thing is that ds doesn't sit and play in any way at all, unless we actually play with him, and even then it will usually only be one small repetitive thing -hold ds's hand, put shape in hole, open box, hold shape in hand put in box etc etc. Fortunately we have the dds who will play with him ( a little bit!), but it is also this that takes us away from the dds, because he needs constant attention.
Do you know anyone in rl with a child with sn? I found things a lot easier when I got to know a lady in town who has a ds a similar age to mine with a chromosomal disorder, we go to music and swimming together, and it helps to see them both having a great time despite not being able to walk or talk etc.
Of course I also have friends with nt dcs of a similar age, and sometimes it is hard, but we were friends before having ds and so have come on this whole journey with me.
As far as the autism thing goes, I suppose I feel with ds this wouldn't be apparent until he was older, as he in so many ways is still such a little baby (well, big baby!). Have you started any threads on here about early signs of autism in young/delayed children? It does seem as though there are a lot of very knowledgable people on MN wrt this area of SN. There is also a very good private paed who has been recommended on here, if you are very conerned that your dd is showing any signs.
Sorry this is such an enormous post, but you sounded so lost and worried, and I think we have all felt that way at some point on this bumpy road.