I had to dash off without completing the post.
I am surprised that someone who has brought up three children in France from when they were tiny and they are effectively French. French is their first language, they have French culture, know French history and know little of anything British. But they are British. But were born in Jersey and are not, therefore,, EU citizens and at 21 fall out of the cover of my EU citizenship did not apply for them to have French citizenship many years ago.
They may be British through their Channel Island birth - but Jersey, as stated above, has 'special status' and is not a member of the EU. They have grown to adulthood (which is surely at 18, not 21) therefore this mother/grandmother presumably knew all this and did nothing over the 20 years to regularise their status in France.
French citizens are entitled to hold dual or multiple citizenships and have been for several decades. In addition, France denounced a Council of Europe proposal that attempted to reduce cases of multiple nationalities
Since the British Nationality Act of 1948, the UK does not restrict its citizens from having other nationalities.
As they are British but effectively French, they could have had dual nationality all those years.
Excuse the copying and pasting, but it would have been more confusing to have inserted links.