Both sides acting recklessly and without regard for the safety of civilians, Éamonn Meehan warns.
Death of five children on Thursday follows killing of four boys, aged between nine and 11, on Wednesday.
UN Security Council urged to immediately take all necessary measures to end new cycle of violence in Israel and Palestine.
Concern over attacks by Israeli military on Gaza's civilian population and firing of rockets by militants from Gaza on Israeli civilians.
"We pray that the visit of Pope Francis to the Holy Land will reinforce hope in the region."