The trouble is, you don’t want to negotiate flexibility, you want to carry on as you are:
being late for work
being the sole carer of 3 young SEN kids while ‘wfh’ after school.
The situation does not appear to be working for your employer, hence the meeting.
Sorry OP, but you stated your case in the opening post and do not appear to actually be open to any solutions that are not the status quo.
While I sympathise with your situation, I do think you will have to attend the meeting in a proactive manner.
You need to come up with some solution that your employer can support where your team does not fall apart and you are not caring for 3 children while wfh.
Perhaps a slight team restructuring could ease things while you are not on site? Suggest someone steps up slightly on occasion, as part of their career development strategy etc.
it seems clear that your employer will not settle for any option where you are both working and caring for your children - and that is not unreasonable of them.
The problem with posting in AIBU is that some people will think you are BU.
A child being a flight risk can be managed successfully in suitable childcare, if they agree to take your child, they can manage the situation - but I do appreciate your reluctance.
You need a solution to put to your employer - not a shoulder shrug and ‘what do they expect me to do’…