Working Tax Credit (WTC) is to help people with children, or people with a disability, or people over 25 working full-time, who don’t earn very much. Those on working tax credit will not be affected by the benefits cap.
People with Down’s syndrome may get Working Tax Credit, if they work and are not on Employment and Support Allowance.
For details, go to Working Tax Credit on GOV.UK. You claim Working Tax Credit at the same time as Child Tax Credit.