Make a time plan to ensure all children in the group will be able to complete 40 sessions, with one 30 min session per day, 4-5 days per week during 8-10 weeks.
A fixed time for training each day usually helps, and the morning is a good training time for many people.
But the most important thing is that you have a plan that works for your group and organization, and makes it realistic to follow through with the whole training.
If you're working with Vektor in school, remember that Vektor is not intended to be a replacement for your ordinary math classes.
In this video, you can get some tips and ideas related to time planning for Vektor - as well as other helpful advice for how to have Vektor work as smoothly as possibly in your school.