Thousands of years ago a group of tribal savages concocted legends about how their imaginary deity promised them a magic land in the middle of a desert, and how they should commit genocide against the people living there. Some of the descendants of these savages—Jews—ended up settling there, getting conquered, reconquered, scattered, and absorbed into other cultures.
Flash forward to the 20th century. Some Jews, living in Western countries, who believe in the surviving religious tradition of these ancient tribal savages, “zionists,” want to resettle this supposedly magic land. In many such countries, Jews were treated horribly, which no doubt motivated some of them to leave.
The problem was that other people, Arabs (Palestinians), were now living on that land. Also, the British government colonially controlled the land. The Arabs didn’t want a lot of foreigners coming into their land all the sudden and taking over. So the British, to appease the Arabs, limited the number of Jews they let in.
Some of the Zionists settled there peacefully. Others got into conflicts with the Arabs, often because the Arabs attacked them. Still other Zionists became terrorists, carrying out vicious “revenge” attacks against Arab civilians and bombing British soldiers and property.
After World War 2, which saw the Jews suffer the worst genocide in history, European colonial powers became much more sympathetic to the Zionists and created a Jewish state, Israel, in the middle of this Arab land that other people were already living on.
Unsurprisingly, these foreigners deciding unilaterally that another group of foreigners could establish a country in the middle of their land angered the Arabs living there even more. So the Arabs attacked the new country of Israel. Israel easily won the war and conquered more Arab land. But rather than annex much of that land into their own country—which would give the Arabs there the right to participate in a supposedly Jewish state—the Jews kept the Arabs under military occupation.
The Arabs, hopelessly outclassed military, resorted more to terrorism. The Jews responded to terrorist attacks often with military force. The cycle continued for decades, with occasional wars where Arab states threaten Israel and Israel uses overwhelming military force to win. Although the Jews do not tend to directly target civilians as the Arab terrorists do, they end up killing many many more civilians through superior military technology.
The imbalance in civilian deaths, along with the basically apartheid state of affairs where Israel militarily controls millions of people who have no representation in Israeli government, enrages the rest of the Arab world and causes the majority of countries in the world to condemn Israel on a regular basis. The main exception is America, which has a disproportionately powerful Jewish lobby and a huge political bloc of insane evangelical Christians who believe it is their religious duty to support Israel because doing so will hasten the end times. Meanwhile, many Israelis believe they have the right to militarily occupy Arab population centers because they require “security” against Arab attacks.
I guess that’s probably too long and complicated for a child to understand. Of course, part of the problem with this conflict is that so many people tend to look at it in black and white terms as if they were children.