I agree with Neil. The best behavior incentive for parents is for the kids to want to be there and be there on time. Instead of doing 'busy work, attendance, checkin etc. how about something short and fun and start it righ on time.
the kids who come late will still get the full workshop but will hear about a fun activity, game song, that they missed. Maybe an 'energizer'.
Reward for those who are on time, incentive for those who are later, and the message is not: " it's okay to be late because they don't do anything the first 10 minutes anyway."