Would really appreciate help correcting my code to get this working!

I am attempting to create a Visualforce page where users can add multiple child records at once. I want to add a list button to the related list on the parent object. Both are custom objects.

The parent object in this scenario Sessions__c and the child object is Class_Enrollments__c. Not sure if this is applicable, but Groups_and_Classes__c is the parent of Sessions__c.

I am new to development, so I reworked some code based on code I found online. Basically, I want users to be able to click a "Add Class Enrollments" button on the Class Enrollments list on the Session record. I want this to take them to a Visualforce page where they can add as many rows as needed, populating the Youth Name, Attendance, and Participation Note fields.

I originally built this with a custom controller, and the visualforce page looked great, except the delete row function was not actually deleting the row. However, when trying to add the button to the page, I realized I have to use a Standard Controller. I've spent hours researching this at this point and have been unable to successfully add the standard controller to the visualforce page and use the controller I created as an extension. I've modified my controller to reference the standardsetcontroller, but then I get a "compile error=unknown token: Id". Basically, I'm so new to this that I'm in over my head. Could anyone please help me correct my code? I would greatly appreciate it.

VF (not edited to reflect ways I tried to add standard controller):
<apex:page controller="AddingEnrollmentsController">
    <apex:form>
        <apex:variable var="rowNum" value="{!0}"  />
        <apex:pageBlock >
            <apex:variable var="rowNum" value="{!0}"  />  
            <apex:pageBlockTable value="{!Class_EnrollmentsList}" var="CE">
                <apex:facet name="footer">
                    <apex:commandLink value="Add" action="{!insertRow}"/>
                </apex:facet>
                <apex:column headerValue="Youth Name">
                    <apex:inputField value="{!CE.Youth_Name__c}"/>
                </apex:column>
                <apex:column headerValue="Attended?">
                    <apex:inputField value="{!CE.Attended_Class__c}"/>
                </apex:column>
                 <apex:column headerValue="Participation Level">
                    <apex:inputField value="{!CE.Participation_Level__c}"/>
                </apex:column> 
                <apex:column headerValue="Comments">
                 <apex:inputField value="{!CE.Comments_for_Treatment_Team__c}"/>
                 </apex:column>            
                <apex:column headerValue="Delete" >
                    <apex:commandLink style="font-size:15px; font-weight:bold; text-align:center;color:red;" value="X" action="{!delRow}">
                        <apex:param value="{!rowNum}" name="index" />
                    </apex:commandLink>
                    <apex:variable var="rowNum" value="{!rowNum+1}"/>
                </apex:column>          
            </apex:pageBlockTable>
            <apex:pageBlockButtons >
                <apex:commandButton value="Save" action="{!insertClass_Enrollments}"/>
            </apex:pageBlockButtons>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Controller (again not updated to reflect my attempts to add the standardsetcontroller):

public class AddingEnrollmentsController {
    Id SessionId;
    public List<Class_Enrollments__c> class_enrollmentsList {get;set;}
    public Integer rowNum{get;set;}
    
    public AddingEnrollmentsController(){
       Id class_enrollmentsId = ApexPages.currentPage().getParameters().get('class_enrollmentsId');
       SessionId=ApexPages.currentPage().getParameters().get('SessionId');
       class_enrollmentsList = new List<Class_Enrollments__c>();  
       class_enrollmentsList.add(new Class_Enrollments__c());      
    }
    
    public pagereference insertClass_Enrollments(){
    insert class_enrollmentsList;
    Pagereference page=new pagereference('/'+SessionId);
    Return page;
}
    
    public void insertRow(){
        class_enrollmentsList.add(new Class_Enrollments__c()); 
    }

    public void delRow(){
        rowNum = Integer.valueOf(apexpages.currentpage().getparameters().get('index'));
        class_enrollmentsList.remove(rowNum);   
    }
}

Any help is greatly appreciated!!!!