If you want DS to attend full-time, he can, unless the school formally suspends. You don’t need the school to agree. Don’t worry if they do suspend. A formal suspension rather than an unlawful informal exclusion it will a) provide you with evidence of unmet needs, b) force the school to follow due process, c) limit the number of days the school can suspend for, d) allow you to challenge any suspension, and e) ensure DS receives alternative provision once he has reached that threshold. If you haven’t already, it is worth reading the statutory suspension and exclusion guidance.
If DS can’t attend full-time, the LA has a duty to ensure he still receives a suitable full-time education. Have you requested alternative provision? On their website, IPSEA has a model letter you can use.
Request a meeting with the SENCO.
Does DS have an EHCP? If not, request an EHCNA. IPSEA also has a model letter for this. If DS already has an EHCP, request an early review.
If going home between dropping off and picking up isn’t feasible, how far away is DS’s school?
Have you looked at transport?
Even if it isn’t worthwhile going home, is there somewhere locally you can go rather than standing around at school with 2 younger DC?
Have you had 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.
Also, look at your local short breaks offer and approach Home Start to see if they can support you.
Your younger DC will still be able to go to nursery.
Personally, I think the first week of the school year is a bad time to take DS out of school for a holiday, but you can’t change what has already happened. I wouldn’t do it going forward, though. Even if you want to go in term time, there are better times to go, IMO.
Be careful with SENDIASS. Some are good but too many repeat the LA’s unlawful policies.