What you might change if you were to rewrite Romeo and Juliet as a children’s story depends on the age of the children, what you believe about childhood, and what you believe about how children learn.
The two things most often eliminated from children’s versions of stories are sex and violence. No explicit sexual acts take place in the play, and the play is considerably less violent than most modern popular culture. Thus I don’t think you need to censor any of the major plot elements in order to maintain a PG rating, as it were. While some people might consider the double suicide at the end disturbing, if you eliminate it, you lose the main tragic element of the story.
Another consideration is the language. It is one of the great glories of the play. Just as you don’t build muscled by lifting a single sheet of paper, you don’t improve language skills by reading only simple monosyllabic prose. What you might consider is using somewhat simple language for exposition, but keeping much of Shakespeare’s original dialogue and using footnotes, hyperlinks, or appendices to help children learn unfamiliar vocabulary.