I'm not sure if there is an easy way to do this with the Google API, but you can achieve this with simple CSS;
Instead
['Alice', 'Mike', ''],
You can write
['<div style="height:50px;vertical-align:top">Alice</div>', 'Mike', ''],
If you want, you can also write JavaScript code to calculate the height of the cell and assign it to the cell tag.
EDIT: If you want to vertically align the TD element, you can write your own CSS;
.google-visualization-orgchart-node {vertical-align:top;}
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