The Crystal is the Heart of the Blade. The Heart is the Crystal of the Jedi. The Jedi is the Crystal of the Force. The Force is the Blade of the Heart. All are intertwined. The Crystal, the Blade, the Jedi, you are one.
The Solo twins Jacen and Jaina are born in the Thrawn trilogy. Anakin (Solo) is born in Dark Empire.
Jacen and Jaina first join the adventures as mere 2 year olds in the Jedi Academy trilogy.
Both they and Anakin take on much larger roles, while still younglings under the age of 10, in the Crystal Star and then the Corellian Trilogy.
Being the grandchildren of Darth Vader/Anakin Skywalker, niece and nephews of Luke Skywalker and children of heroes, they are constantly in danger of kidnapping. Just as Anakin Skywalker, all three children have a tendency to find themselves in positions to influence Galactic events, often without realizing they are doing so.