There are well understood, well reasoned and documented reasons as to why a 12 year old should not be playing with a senior team. The reasons relate to a number of factors, emotional development and maturity, physical development and appropriate competition, technical and skill development.
Nothing to do with tall poppies, everything to do with the best practice to assist a player to become as good as they can.
Bringing a child so early into senior sports of any sorts stunts their development.