OK kiddo, a long long time ago, in the olden days of 1603, England and Scotland used to be two separate countries. They each had their own kings and queens, and sometimes they fought with each other. But one day, England and Scotland decided to become one big country called Great Britain!
To make this happen, they needed to agree on some rules for how the country would be run. So, the leaders of England and Scotland got together to make a special agreement, called the Union of England and Scotland Act 1603.
This agreement said that England and Scotland would now have the same king or queen, who would be in charge of both countries. It also said that they would have a united government, with one parliament made up of people from both England and Scotland.
This was a big deal, because it meant that England and Scotland were no longer enemies, but friends who would work together to make their new country even better. And that's how Great Britain came to be!