Why am I getting these undefined index errors after submit?

I'm trying to display and update the showChild using both select and update queries. I'm requesting the ChildID variable from previous page, I'm pulling the data of the child via the children table and outputting the results. If the user wants to change the venue name and time of the child they can do so from a drop box from pulled from the venue table. Once they save the data then $VenName and $VenID values are updated on the children table and the page is refreshed with the new venue details.

I can display all the data perfectly but once I submit I get these Undefined index errors

Notice: Undefined index: ChildID in /homepages/19/d796710461/htdocs/podmin/beta/showChild.php on line 16

Notice: Undefined index: ChildID in /homepages/19/d796710461/htdocs/podmin/beta/showChild.php on line 57

Notice: Undefined index: VenID in /homepages/19/d796710461/htdocs/podmin/beta/showChild.php on line 59

Here is the full code of showChild.php. I know that I haven't yet done a prepared statement for listing the venue names which I will do later. Hope some of you geniuses can see where I'm going wrong..

ini_set('display_errors', 1);

session_start ();
//Connect to the database
include ('conn.php');

//Call functions
//include ('functions.php');

//Lets create the venue ID session variable 
$id = $_REQUEST['ChildID'];
$_SESSION['id'] = $id;

//Prepared statement and bind  
$sql = $link->prepare("SELECT * from children WHERE ChildID = ? "); 
$sql->bind_param("i", $_SESSION['id']); //declare the variable as a string or integer


$result = $sql->get_result(); // get the mysqli result
$row = $result->fetch_assoc(); // fetch data   

//Store associated venue into a session
$_SESSION['Venue'] = $row['Venue'];
//$_SESSION['VenID'] = $row['VenID'];


if (isset($_POST['update'])) {

    $edit = $link->prepare("UPDATE children SET Venue = ?, VenID = ? WHERE ChildID = ? LIMIT 1");      
    $id    = $_POST['ChildID'];
    $Venue = $_POST['Venue'];
    $VenID = $_POST['VenID'];

    //$id = $_POST['id'];

$edit->bind_param('sii', $Venue, $VenID, $id); // 's' specifies the variable type => 'string'

    echo "<script language='javascript'>;
    alert('New venue has been updated ');

    //close the query


        <div class="contact-clean">
            <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
                //Convert DOB to  d/m/yyyy
                $date1 = strtr($row['ChildDOB'], '/', '-');
                $DOB = date('d-m-Y', strtotime($date1));
                //Convert Date registered to  d/m/yyyy
                $date2 = strtr($row['DateReg'], '/', '-');
                $Reg = date('d-m-Y', strtotime($date2));
                //Work out how many days registered
                $now = time(); 
                $reg = strtotime($date2);
                $datediff = $now - $reg;

                <h2 class="text-center"><?php echo $row['ChildName'];?></h2>
                <h6 class="text-center">DOB: <?php echo $DOB;?></h6>
                <div class="form-group"><label>Status</label>
                    $paid = $row['is_active'];

                        if ($paid ==1) {
                            echo '<h2 style="color:Green; text-align:center;">Active - Payment up to date</h2>';
                        } else if ($paid == 0) {
                            echo '<h2 style="color:red; text-align:center;">Not active - No payments received</h2>';
                        } else if ($paid == 2) {
                            echo '<h2 style="color:blue; text-align:center;">***Waiting List***</h2>';

                <div class="form-group"><label>Joining Date</label><input class="form-control" type="text" name="ChildDOB"  disabled value="<?php echo $Reg;?> (<?php echo round($datediff / (60 * 60 * 24));?> days ago)"></div>
                <div class="form-group"><label>Emergency Contact Number</label><input class="form-control" type="text" name="EmTel" placeholder="Emergency contact Number" disabled value="<?php echo $row['EmTel'];?>"></div>
                <div class="form-group"><label>Important Information</label><textarea class="form-control" name="Info" placeholder="Important Information" disabled><?php echo $row['Info'];?></textarea></div>
                <div class="form-group"><label>Paypal ID</label><input class="form-control" type="text" name="paypalD" placeholder="Paypal ID" disabled value="<?php echo $row['paypalID'];?>"></div>
                <div class="form-group"><label>Venue</label><select class="form-control" name="Venue"><optgroup label="Please select a venue">
                <option><?php echo $_SESSION['Venue'];?><option>

                        $sql1 = "SELECT * FROM venues ORDER BY VenName ASC";
                        $result = $link->query($sql1);

                        if ($result->num_rows > 0) {
                        $_SESSION['VenID'] = $row['VenID'];                        
                        // output data of each row
                        while($row = $result->fetch_assoc()) {
                        echo " <option> " . $row['VenName'] . " " . $row['VenTime'] . "</option>";
                        } else {
                        echo "No venues listed";

                    <div class="form-group"><input class="form-control" type="hidden" name="VenID" disabled value="<?php echo $_SESSION['VenID'];?>"></div>

                <div class="form-group"><button class='btn btn-primary' type='submit' name='update'>Save</button>
                <a href='<?php echo "paymentAuth.php?ChildID=". $row['ChildID'] . ""; ?>' data-toggle="tooltip" title="Payments"><button class="btn btn-primary fas fa-pound-sign" type="button" name="Payments"></button></a>
                <a href='<?php echo "showParents.php?UserID=" . $row['UserID'] . ""; ?>'><button class="btn btn-primary fas fa-users" type="button" name="Parents"></button></a>                    
                <a href='<?php echo "delChild.php?ChildID=" . $row['ChildID'] . ""; ?>' data-toggle="tooltip" title="Delete Child"><button class="btn btn-primary fas fa-user-times" type="button" onClick="return confirm('WARNING! Are you sure you want to delete this account? This cannot be undone')" name="Delete"></button></a>
                <h6><a class="fas fa-hand-point-left" onclick="goBack()" style="padding-top:20px;"> Go back</a></h6>


    function goBack() {

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<script src="assets/js/bs-init.js"></script>
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