This comment from Crush is so true. If I had a dime for every time "A" team parents crapped on "B" team kids I could retire. Given the quite mediocre level of most top teams, it's comical. Especially in the top national leagues. Parents aren't exactly excited about your kid taking playing time away from Johnny's Division 1 College highlight reel.
As to the difference between A and B teams, generally speaking the first team should present a higher paced, technically advanced, physically and mentally challenging training environment to develop their game. In a quality club, the age group presents opportunities for select kids from the second teams to train and/or guest play with the first team. This is often promised, yet seldomly delivered.
To be honest, if your kid isn't watching/studying the game, and putting in work on their own time to improve their skills, then the whole narrative of expecting to play on the "A" team is a moot point. Expecting coaches and teams to develop your players is understandable, but from my experience, even in top national teams, half or more of the kids don't even watch the sport or put in time on their own training their skills and playing the game. So what's the point? Just setting yourself up for disappointment and empty pockets.
The absolute best teams I have developed have been second teams with a great supportive parent group, kids who love to play with each other, and push each other as a group regardless of skill level. It really depends on the situation and the quality of the group/coach/team.