I think you can place them in an array then reverse them. this will be an example below

    function addTask() {
        // Declaring Variables
        var item = document.getElementById("text_field").value;
        var text = document.createTextNode(item);
        var list_element = document.createElement("li");
        var checkbox = document.createElement("input");
    
    checkbox.setAttribute("type", "checkbox");
    list_element.appendChild(checkbox);
    list_element.appendChild(text);
    document.getElementById("todo-list").appendChild(list_element);
}
    function reverseElement(){
 var reversedElement = Array.from(document.getElementById("todo-list")).reverse()
} 

Use prepend instead of append.

Here is the working example:

function addTask() {
    // Declaring Variables
    var date = new Date;
    var item = document.getElementById("text_field").value;
    var list_element = document.getElementById('tasks');
    var li = document.createElement("li");
    li.innerText = item + '\t|\t' + date.toISOString();
    
    list_element.prepend(li);
}

<input id="text_field" />
<button onclick="addTask()">Add</button>
<div>
<h1>Tasks</h1>
<ol id="tasks"></ol>
</div>