(WWJD? How about the parable of the late coming "Laborers in the Vineyard" as a clue, or the Lost Sheep.)
Another question: Why should a church think it has to play "cop" on those who arrive late? We are not the public school. Do we punish late arrivers in worship or for meetings? Sounds like a double standard for kids.
Encouragement yes, restrictions NO.
<>< Neil