Child disability premium on top of Housing Benefits

As you're claiming UC you will only be claiming help with the rent through housing benefit if you're either living in supported or temporary housing. If you don't then help with the rent will be paid through UC known as housing element. There's no disability premiums payable when claiming UC, this includes the severe disability premium. 

If you privately rent then you'll be entitled to the local housing allowance for the area you're living. How much you're entitled to will depend on how many bedrooms you're entitled to. You can see the LHA rates here. https://lha-direct.voa.gov.uk/

If you're not sure how many bedrooms you're entitled to you can check here. https://lha-direct.voa.gov.uk/bedroomcalculator.aspx