Frequence Jeune is very generous IMO :
- a minimum of 1000 miles for every domestic segment with AF (even with N fares)
- a minimum of 1000 miles for European segments with AF (do not know for N fares)
- a 4000 miles bonus every 6 segments on AF, regardless of the period
If your child will fly mainly on (cheap) domestic tickets, FJ seems better.
If you think she (IIRC) will be able to earn enough status miles to be granted elite and therefore earn bonus club miles that will be higher than the 4000 miles FJ bonus, I suggest to go for FB.