You can't expect them to pay for your childcare on top of your fostering allowance. It's up to you if you work but you need to keep it within school hours, I can't see any SW agreeing to your dd (assuming you are fostering her) coming home and being at home alone at age 11. It might seem unreasonable but I'm sure you know that the rules for fostered children are much stricter than if she was your birth child.
You might be able to claim tax credits for the childcare costs, as fostering allowances are not taxable income.

I've worked part-time for the last year as I had to take dd to school and pick her up from school (40 minutes away).

Now she's moved to secondary I wanted her to be able to be at home but SS said no.

I've been offered work which will mean she will need to go to a childminder after school til I finish at 6.

They are saying they won't pay for it and I wondered if other foster carers worked because I've certainly no intention of staying part-time til they think she's old enough to come home on her own.