We have a large age range (3 - 16), a small number of students (6 - 8) and a short Sunday school time (usually 45 min max), so we have changed our opening structure to a simple candle-lighting, song and prayer.
We save discussion for the individual lessons, which allows the leader to tailor it to the appropriate age range and maximize their time (e.g. discussion while things are cooking instead of at the beginning) but kept the common opening so that all of our kids get to know each other and feel comfortable with each other.
We keep the song and prayer the same for the entire rotation, so that the kids know the song fairly well by the end.
Our minister tries to incorporate as many of the Sunday school songs in her worship as she can, allowing even the non-readers to have some songs that they know during worship.