Yes I agree MartavTaurus
"Because that is the whole concept of St Valentine's Day. Romantic love is historically narrow and socially outdated"
We are human beings not automatons. What a cynical attitude to deem romantic love outdated. It's a timeless heightened emotion that often is one of the great highs and sometimes lows of the human condition. Otherwise we enter the horrible world the Soviet Union created of the "personal life is dead". perhaps that way of thinking resonates with some, personally I think it's a very negative point of view. The gesture of romantic love by way of sending a card is still alive and kicking present in the shops certainly amongst the cards I've scanned. Albeit what surrounds all of that is big business and like all special days in the calendar its long since been turned into a commercial enterprise. The range of cards, everything from hearts and flowers to rude sexualised content, are solely aimed at loved ones, or someone in the giver's sights, or possibly sent as a joke.
I think the premise of a Valentine card sent from a child, sweet as it briefly maybe, our children need to know the love they have for their parent is entirely different from that they will have for a prospective partner. I remember trying to explain that to my then 4 year old who said "when I grow up I'm going to marry you mummy" At that age children have no concept of prospective romantically involved relationships. Maybe it's something that would be deemed quite sweet in the moment where adult life is beyond their comprehension. Definitely something I wouldn't encourage once they grew out of that stage.
As everybody else has stated, it is totally inappropriate for "A card with romantic intent" to be sent to some unknown adult person from a child.