For anyone looking for this information here it is as of June 2018
Here’s the Child Maintenance Calculator and Helpline <span style=”color: #0b0c0c; font-family: nta, Arial, sans-serif; font-size: 19px;”> 0800 171 2033</span>
https://www.cmoptions.org/en/calculator/calculator.asp
and the tax credits helpline number is <span style=”color: #222222; font-family: arial, sans-serif; font-size: 32px;”>0345 300 3900</span>
and website https://www.gov.uk/child-tax-credit/new-claim
you may be in a universal tax credit area, you can find out on the above link.
you can talk to them first they are friendly and want to help, say you want to make an application, make a list of all details you need, then get together before going online.