Only the six under 18 would qualify for Child Tax Credit or for the Child Element in Universal Credit. As in my OP £500 looks about right on tax credits and JSA. Because of the Cap she'd lose all but 50p of Housing Benefit but would still need to pay the rent in full and, in most places, a contribution to Council Tax.
She'd be expected to look for work but only outside school time as the youngest is under 13. That doesn't stop her taking work and paying for childcare, with support in UC or Tax Credits, but the cost would likely be prohibitive.
If however she could find 16hours a week @ £9/hr then she'd be a lot better off as she'd escape the cap.
OTOH with 6 school age children, even without pandemic related issues at school, would she have satisfactory attendance?