George is three and his sister is only 11 months, but Wills and Kate are going to have another baby. If you're royal, you need not only heirs, but spares. Primogeniture means the eldest child of the reigning monarch is next in line for the throne. However, the system doesn't always work perfectly. When Edward VIII abdicated, the "reluctant king," had to step up. When George VI died, Elizabeth II became queen.
Obviously some children, even when raised and trained to take the mantle, may be unwilling or unable to handle the top royal role. And as attitudes toward monarchy continue to evolve, who knows whether George will one day be King? At the moment, he's just two steps away. Charles in next in line, and after him, George's father William.
It's possible that George won't want the job, even when his turn comes round. Perhaps, Charlotte, like her grandmother Elizabeth II, will one day reign as queen. But will the next British rulers also reign over Canada, even symbolically? I find it hard to imagine, yet even harder to imagine the political process that would be needed to make such a change.