I have a child with autism and I'm a lone parent too so I really sympathise, other parents (even lone parents) often just don't how impossible it is to deal with a child with SN. I take my hat off to you for coping with two dc with SN, and managing to work so far as well!
I have relied on benefits since my child got DLA. I get higher rate care, which means our child tax credits are higher than the normal rate, and I get income support, carers allowance, council tax benefit and housing benefit as well. It gives us a comfortable amount to live on (I used to be on income support just as a lone parent, without the disability/carer top-ups before DS was diagnosed, and it was a real struggle to live off those amounts). We don't get maintenance for various reasons.
In practical terms, DS can't access any form of childcare due to his behavioural issues, plus he is also out of school regularly, so working is not an option for me. I also need to use the time he is in school to do errands and housework, which I can't do when he gets home from school due to the high level of supervision he needs. Life as a carer of a child with SN is so different from looking after a typical child who can be dropped off in any form of childcare, be left alone for short periods, self-occupy in another room while a parent does housework etc.
You can claim income support for as long as you get carer's allowance - even when your child is an adult, there's no age limit as with lone parents. You have to go for work focused interviews every year but it's nowhere near as bad as signing on, our Jobcentre is quite understanding that I'm not able to work with my DS's needs.
We also get extra amounts of money, e.g. Family Fund give us a grant once a year which means I can buy things to help DS like a games system, and we get cash from our short breaks team which pays for activities for him, and we've had funded holidays in the past as well.
We've been turned down for support from Social Services as well, so having an autism diagnosis is no guarantee of support. And yes, there is more specialist schooling for autism, but compared to the rates of diagnosis there is just not enough, so many children with autism can't get a place, plus the specialist support is not always appropriate for that specific child. But I can understand that it must be more isolating if your child has a less common diagnosis. I get a lot of support from other parents of children with autism and there is a lot of information out there.
I'd suggest getting in touch with Contact A Family, as they have a useful helpline for parents caring for children with all sorts of disabilities and they can give benefits advice. CAB is good and reliable, but they are so difficult to get an appointment with these days, and they don't specialise in families with disabilities, whereas CAF do. I've also had good support from the National Autistic Society - there is probably a similar support organisation for your children's disabilities which should be able to offer help.