If your young students are always predictable and super well behaved, congratulations! You don't need to read any further. You have earned a nap! (And maybe you need to start your own blog for the rest of us to follow!)
But for those of us who aren't so lucky, we need to talk about the importance of always being prepared and ready to change your plans mid-class. We never know what kind of child we will get on a given day. They may be happy, tired, hungry, stressed, need a hug, be wound up, want attention, or any combination of physical or emotional states. (And don't even get me started on their parents! That's a whole post in itself!) It is not your fault, but it is your responsibility to know how to cope. Recognize when you may need to prevent problems or change your plans, if necessary, to have the most successful class.
For the littles (ages 2-5), I always plan one or two extra, alternate activities in my lesson plan, and bring in all props, music, and visuals necessary for that extra activity. I even mark it as "Extra 1" and "Extra 2" on my lesson plans (clever, huh?) This makes adapting seamless. You don't need to stress, try to come up with something on the spot, or frantically have your assistant or office manager find the right prop or music for a spur of the moment plan change. You just adapt. No one knows you have changed your original plan. The kids don't know. The parents don't know. You just do it and move on. Hopefully the alternate activity helps you, whether it is to refocus students, regain control of the class, or fill the empty block of time that was created when you had to abandon an activity.
Being prepared does take a little bit of extra effort, but it is well worth it to keep those classes running as smoothly as possible and your sanity in tact. With your experience in recognizing the need for a change in activity, and having the necessary visuals, music, and props ready to go, you will remain calm, flexible, and manage your classroom like a BOSS!