I assume the lack of needing a sign in for CBBC and CBeebies is to avoid data protection headaches with kids needing accounts, parental permission to sign up etc?
Actually you can sign in:
BBC iPlayer is changing and in some places you will need to sign in to watch TV shows.
Signing in is really easy:
If you already have a BBC account (for example to comment on the CBBC website) just keep using that one.
If you don't already have a BBC account, you will need to create a new account.
]If you already have an account that you use for the CBBC website then use that to sign in to the BBC iPlayer app
Mind you its as clear as mud because further down the page is a "full CBBC account" link which appears to contradict the earlier info on the page.