String representation of Tree
Some suggestions (by far not complete as the other answer(s) show):


You don't need to specify the type twice:

protected List&lt;Tree&lt;T&gt;&gt; children = new ArrayList&lt;Tree&lt;T&gt;&gt;();
protected List&lt;Tree&lt;T&gt;&gt; children = new ArrayList&lt;&gt;();

Create a new parameter maxDepth to limit the function nesting level.
It should be set to a default value to follow the principle of least astonishment: Nobody would expect his program to crash just because he triggered the string conversion somehow.
With very big trees the nested function calls can become a problem.
But I wouldn't worry about it for two reasons:


The function is not named archiveTree. As a user, I wouldn't expect it to handle each and every scenario of a tree.
The default value maxDepth should reasonably prevent out of memory scenarios as long as the user doesn't manually provide disproportional values. If he does, it's his fault and not yours anymore.

You sometimes have inconsistent spacing, e.g.:

buildString(sb,prefix, true);
buildString(sb, prefix, true);

You could specify the capacity argument with a guessed value to the constructor of StringBuilder to prevent some unnecessary capacity allocations. However, as long as you haven't identified this as a noticeable bottleneck of your application, I would consider it to be premature optimization.