Here are two ways to think about a moderate- and vigorous-intensity aerobic activity:

On a scale of 0 to 10, where sitting is a 0 and the highest level of activity is a 10, moderate-intensity activity is a 5 or 6. When your child does moderate-intensity activity, their heart will beat faster and they will breathe much harder than when they are at rest or sitting. Vigorous-intensity activity is a level 7 or 8. When your child does vigorous-intensity activity, their heart will beat much faster than normal and they will breathe much harder than normal.
Another example, is when your child walks to school with friends each morning. They’re probably doing moderate-intensity aerobic activity. But while at school, when your child runs, or chases others by playing tag during recess, they’re probably doing vigorous-intensity activity.