Typically between the ages of 2-4 years old, children will show a preference for one hand over the other. This is called hand dominance, and is the point at which you could say "I'm left-handed" or "I'm right-handed."
Developing hand dominance is an important step in development for a variety of reasons, but especially for efficiency. By developing hand dominance, a child will be better--faster, more precise--at completing certain tasks (like writing or cutting). These types of hand movements will become more automatic, meaning they require less brain power and less focus. This frees up the brain to focus on other things.
As your children develop, you should be observing the changes that are happening. Switching hands is a normal part of this process, but at a certain point (typically before entering kindergarten) a child should show preference for one hand over the other. By taking observational notes with photos or work samples to support, you can keep track of each child's progress. If a child is getting ready to enter kindergarten and still hasn't shown preference for a dominant hand, it might be time to raise the flag and alert the parents.
In the meantime, giving your kids the opportunity to work on all skills related to pre-writing will also support hand dominance. As with most aspects of development, they are very much interrelated.