Consent letter for minor

I have submitted visa application form my kid and wife. In the appointment confirmation letter of my Kid who is 6 months old, there is point mentioned as below:
"If you are under 18, one of your parents or guardians must give their permission in writing to you travelling to the UK. Please include this letter of consent with your supporting documents"

what is this all about? Does any one have submitted this consent letter?

My daughter will be travelling either with me or my wife or we all together travel, still do we need to submit consent letter?

What should be the format of letter? Does it need to be notarized or some kind of affidavit is needed?

Can my wife herself give a letter on a plain paper putting her signature alone?

Please advise as I am bit confused.

Thanks.