You can get permission to deal with someone else’s tax credits. The type of permission you need depends on what you want to do.
contact HM Revenue and Customs (HMRC) about someone else’s claim - as an authorised intermediary
take responsibility for someone else’s tax credit claim when they cannot manage their own affairs - as an appointee
claim tax credits for both your child and their baby
For further information on claiming tax credits for someone else, see GOV.UK's website.