I can run through a rough calculation, but I can tell you now, that you won't get the full 70%, sorry. Tax credits are made up of elements. For 2017/2018, they are:
Working Tax Credit
Basic element £1,960
Couple and lone parent element £2,010
30 hour element £810
Disabled worker element £3,000
Severe disability element £1,290
Childcare Element of Working Tax Credits:
Maximum eligible cost for 1 child £175
Maximum eligible cost for 2 or more children £300 per week
Percentage of eligible costs covered: 70%
Child Tax Credit Rates
Child Tax Credit family element £545
Child element £2,780
Disabled child element £ 3,175
Severely disabled child element £ 1,290
Tax credits income thresholds and withdrawal rates
Income threshold £6,420
Withdrawal rate (%) 41%
Threshold for those entitled to Child Tax Credit only £16,105
Income rise disregard £2,500
Income fall disregard £2,500
For your circumstances, you would get the following allowances:
Working Tax Credit
Basic element: £1960
Couple and lone parent element: £2010
30 hour element £810
Total=£4780
Childcare Element of Working Tax Credit:
£10500×70%=£7350
Total=£7350
Child Tax Credit:
Child Tax Credit family element £545
Child element £2,780×3=£8340
Total=£8885
Your income: DH £33k, you £9.5k; Total=42.5k
Income increase disregard: £2.5k. Assumed total income for Tax Credits: £40k
Allowance: £6420. £40,000-£6420=£33,580.
Income taper: 41%. £33,580×0.41=£13,767.80
£13,767.80 is how much you will have deducted from your tax credit award as excess income.
So, your award would be:
Working Tax credits £4780 (£13767.80-£4780=£8987.80)
Childcare Tax Credits £7350 (£8987.80-£7350=£1637.80)
Child Tax Credits £8885 £7247.20 (£8885-£1637.80=£7247.20)
So you actually wouldn't get anything towards your childcare in a way that you would see, but it would raise your child tax credit award by £7000, so that your deduction due to income is less of a cut.
In other words, if you take on this work and find a way to do without childcare, you'll get no tax credits at all. If you take on this job and need childcare, you're likely to find that you're going to get the value of 70% of the childcare in child tax credits, but no Childcare Working Tax Credits, because by paying for childcare you just dip your income back into the eligible range.