Fast, but that assumes that the children are 'all part of the same foortball team', which is not the case.
Using that analogy, the first team live, work and train as the first team. Yes, they are joind by trainees and subs and others are called up from the second team on some occasions, but essentially they are the first team.
Some are in the Sunday League team. Although sometimes a secoind team player will be with them, and sometimes one of them will play with the first or second team, they will, in general, live, work and train as the Sunday League team.
The experience of the first team is not diluted by the Sunday league team, because their points of contact are fairly limited. However, simply by having all the teams based in the same training ground (school), players can be promoted, relegated and switched around easily - and importantly, they all benefit from having the same facilities to work in.