Is it necessary to warn children about their envious brother?

It is not permissible to accuse someone of envy based solely on suspicion. If you are certain of someone's envy, the remedy is to advise them wisely, explaining the danger of envy and the obligation to fear Allah, and to say: "What Allah wills, there is no power except with Allah" (Ma sha' Allah, la quwwata illa billah). If they do not take heed, there is no harm in warning against them and advising caution against their evil. This is not among the forbidden backbiting, because warning against the evil of an evil-doer is not included therein.