my $people = {};
for my $m (keys %$input) {
    add_person($people,$m, parents => $input->{$m}{parents});
    for my $p (@{ $input->{$m}{parents} }) {
        add_person($people,$p,children => $m);
    }
}

sub add_person {
    my ($people,$name,%params) = @_;
    
    if(! defined $people->{$name}) {
        $people->{$name} = { children => [], parents  => [] };
    }
    for my $par (keys %params) {
        push @{ $people->{$name}{$par} }, 
            ref($params{$par}) eq "ARRAY" ? @{$params{$par}} : $params{$par};
    }
}