Do we need a consent letter or document other than the standard passport and birth certificate for children when both parents are also going along?
The consent letter is needed ONLY if the children are travelling alone. If any adult is accompanying the minors then you’ll need to provide specific information about them in your application.
More details at https://www.gov.uk/standard-visitor-visa/if-youre-under-18
Hope this helps.
You do need a letter of consent even if the child is travelling with one parent. My son was travelling with to the UK with both parents. My husband already had a UK visa so only me and my son applied. Luckily I took a no objection letter with my husbands signature on it or they wouldve rejected my application. The lady told me there is a format but there is none on the website. Its better to get a letter and be safe than to be sent back. Nothing lost if you take it.