Football teams have long used children as mascots. The children themselves enjoy it, and it adds a bit of "family value" to the sport.
However nowadays it is something for which the children's parents often have to pay. For a fee a club will offer a mascot opportunity, which involves them getting to meet the players, having a kick around with them, and ending up with a team strip to take home. Some children receive such a mascot opportunity as a birthday present.
International teams will sometimes provide such an opportunity to the physically disabled, or to a child who has suffered in some way eg the loss of someone close to them. David Beckham the great star of the 1990s and 2000s when playing for England, was often accompanied on to the field by a little girl with a portable breathing apparatus.