What support is the school providing?
Does DS have an EHCP? If not, has an EHCNA been requested? If he already has one, what provision is already in it and have you requested an early review? There are other options if the school isn’t suitable.
While sorting more support via an EHCP, if DS is unable to attend school, you could request alternative provision.
What support for his anxiety is he receiving?
Has DS had an OT assessment? Including a sensory OT assessment? Has he also had a home OT assessment?
How are his eating and sleeping?
Some people find the books The Explosive Child and the Out of Sync Child helpful. Some find low demand resources useful. Some find non-violent resistance resources useful.
You can request social care assessments. A carer’s assessment for you and an assessment of DS’s needs. On their website, Contact has model letters you can use. Sometimes there is funding available for courses on how to deescalate and if necessary safely hold/restrain DC.
For DD, have a look at your local young carer’s service and Sibs.
As well as diary looking for triggers, it can help to keep a diary about the wider circumstances to help you spot signs so you can deescalate before DS receives the point of no return, and to know what strategies work/what don’t.