Conditional IF current cat is the smallest/last/end child cat THEN

You can check if the current term has children. If it does, it’s not the lowest, if it doesn’t, it’s the lowest of that branch.

if( is_category() ){
    $children = get_terms( [
        'taxonomy' => 'category',
        'child_of' => get_queried_object_id(),
        'hide_empty' => false
    ] );
    if( empty( $children ) ){
        echo 'this term has no children';
    } else {
        echo 'this term has children';
    }
}

I’ve also set hide_empty to false, which will return empty child terms. If you don’t want to return terms that have no posts assigned, you can remove that argument.