One thing I have learned from working with children is you need to be prepared. When things do not go according to your plan you have to have some backups!
I have found that by having lots of small group activities you can keep the children interested if your first plan does not work out.
Whatever can go wrong will. Keep this in mind while planning your lessons. For example if you plan for the art project to take 20 minutes and it only takes 5 how will you fill the other 15 minutes?
By having extra activities for the children it will help fill in some of these gaps.
Having these extra plans will also help if the children are having trouble with the lesson. The ones who are struggling can have some small group help while the others do another activity.
Always be prepared to keep your class in order you do not want chaos. When in doubt or stuck remember pinterest.