applying for Finace visa who's child has a british passport

Hi,

My fiance is Mexican and has a 3-year-old child from a previous relationship. She wasn't married and never applied for visa to uk. She lived in Mexico and raised the child there. The father was British meaning the child has a Mexican and UK passport. The passport is valid. I meet all the requirements so I'm hoping the application will be simple and that i only need to apply for my fiance and not the child as a dependant. Hoping when arriving at immigration they'll see child passport as well as mothers 6-month visa and they'll be no problems but i'd appreciate any advise. Should we mention in the application she has a child with a British passport. My earning mean there's no problem supporting her but i don't know if needed to be mentioned.

Worth noting the child's father is in support of us coming to UK as wants her to be educated in the UK. i have letter written by him saying as such.