"The children are eating some vegetables."
Translation:Les enfants mangent des légumes.
"Les enfants sont en train de manger des légumes." is expected as one of the answers without introducing "sont en train" earlier in the lessons.
Now that you have been introduced that formulation, here are the details:
In English, to mean that an action is in progress at the time you speak, you use the continuous verbal form, ie verb BE + action verb in the gerund form (-ing). o he is eating means he currently eats
In French, this verbal form does not exist (directly translated “il est mangeant” is incorrect). Therefore, you can translate either “il mange” or “il est en train de manger”, where the construction verb être + en train de + infinitive correctly expresses the English continuous form.