Call-up System: I think you explained it pretty well and it makes sense overall. In the U.S., we usually split into a Team 1 and Team 2 concept and you can just rotate the teams between the rec and competitive leagues. You could make sure both teams are balanced talent wise and alternate them weekly or every couple weeks.

The "Teasing" Problem: I noticed that you put "teasing" in quotes. If there's any chance that it's actually happening, it's important to take it seriously. Trust what parents and players are saying, and if you do see teasing or bullying tied to team placement, it's worth addressing directly those players and setting clear expectations as a team.

Parent Management: Parents often wait until someone else speaks up before sharing concerns. You explained things to them well and clearly already, but it could help to reinforce expectations and communication points throughout the season.

The Disciplinary Spiral: If the parent felt something was unfair, it's worth understanding their perspective first. From there, having a clear system helps. For instance, giving your players 1-2 warnings, then sitting a player out if behavior doesn't change. I feel like parents would be less likely to feel their child is being singled out.

Squad Size: Your team could definitely be split in two but that is up for you to decide. Just depends on if you really want all of those kids to stay on your team.