<?php

$a = array(
    'Alpha' => array('Red' => 'one', 'Blue' => 'two'),
    'Bravo' => array('Blue' => 'three'));

function convert_array($from){
    if(!is_array($from)){
        return false;
    }
    $to = array();
    foreach($from as $k=>$v){
        $to[] = array(
            'title' => $k,
            'children' => convert_array($v)
        );
    }
    return $to;
}

print_r(convert_array($a));

Array
(
    [0] => Array
        (
            [title] => Alpha
            [children] => Array
                (
                    [0] => Array
                        (
                            [title] => Red
                            [children] => 
                        )

                    [1] => Array
                        (
                            [title] => Blue
                            [children] => 
                        )

                )

        )

    [1] => Array
        (
            [title] => Bravo
            [children] => Array
                (
                    [0] => Array
                        (
                            [title] => Blue
                            [children] => 
                        )

                )

        )

)