I am taking my dd to the vigil, but as she is 5 mo. the meaning is not yet an issue.
However, as a preschool teacher, I have had to deal with these types of explanations before (like after 9-11). What we would do in school is hold hands in a cicle and i would explain how we need to send our love all over the world and to Mother Earth to help people to be kind to each other, just like we are wiyh our friends. you could get more specific with exactly who needs to be kind, or instead say that the love is for people to stop hurting each other, etc.
good luck with the conversation, I think its important for kids to experience things like th vigil.