Building nested array with foreach

I am trying to build an associative multidimensional array like this:

array(
    [0] => Array(
        Parent => Mr Smith,
        Children => array(
            Firstmane => Bob,
            Surname => Smith,
            Age => 16,
        )
    )
    [1] => Array(
        Parent => Mr Jones,
        Children => array(
            Firstmane => Davey,
            Surname => Jones,
            Age => 15,
        )
    )
)

My current code looks like this:

foreach($Parents as $parent) {
    $opdata[] = ['Children'=>array(),'Parent'=>$name]; 
    
    foreach($children as $child) {           
        $opdata['Children'][] = [ 
            'Firstmane'=>$code, 
            'Surname'=>$cost, 
            'Age'=>$age,
        ];
    }    
}

However, This puts the ‘Children’ in a new array rather than in the child array of each parent. Like this:

array(
    [0] => Array(
        Parent => Mr Smith,
        Children => array()
    )
    [1] => Array(
        Parent => Mr Jones,
        Children => array()
    )
    Children => array(
        [0] => array(
            Firstmane => Bob,
            Surname => Smith,
            Age => 16,
        [1] => array(
            Firstmane => Davey,
            Surname => Jones,
            Age => 15,
        )
    )
)

I can’t work how to add the child arrays to the correct nested array with the correct parent.

First create, then assign, not the other way around. So in your case the code could look like this:

foreach ($Parents as $parent) {
    $person = ['Parent' => $name];
    foreach ($children as $child) {           
        $person['Children'][] = ['Firstmane' => $code, 
                                 'Surname'   => $cost, 
                                 'Age'       => $age];
    }    
    $opdata[] = $person; 
}

I say: "could look", because it is unclear where a variables like $name, $code, $cost and $age are coming from. I guess it has to come from the $parent and $child variables, but they are not.

I also tried to make the code easier to read using indentation. How you indent is up to you, but smushing everything together doesn't help readability.