Custom Validation TagHelper: Modify Child Element

I am trying to implement server side validation, by changing the class of an HTML child element within a TagHelper. The TagHelper is the "kendo-datepicker", however the current code modifies the class of the "span" tag and I would like to modify the class of the child "input" tag.

After some research it appears GetChildContentAsync may be useful, but all my attempts to use it are failing.

Taghelper:

[HtmlTargetElement("kendo-datepicker")]
[HtmlTargetElement("select")]
[HtmlTargetElement("input")]
public class ValidationErrorClassTagHelper : TagHelper
{
    [HtmlAttributeNotBound]
    [ViewContext]
    public ViewContext ViewContext { get; set; }

    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {

        //output.Attributes.ContainsName("class")

        if (context.TagName == "kendo-datepicker")
        {
            TagHelperAttribute taghelperAttribute = context.AllAttributes["for"];
            if (taghelperAttribute != null)
            {
           
                ModelExpression modelExpression = (ModelExpression)taghelperAttribute.Value;

                ViewContext.ViewData.ModelState.TryGetValue(modelExpression.Name, out ModelStateEntry entry);

                if (entry != null && entry.Errors.Any())
                {
                    output.Attributes.SetAttribute("class", "form-control " + "is-invalid");
                 
                }
            }
        }

    }
}

Current (incorrect) HTML Output:

<span class="k-datepicker k-input form-control is-invalid k-input-solid k-input-md k-rounded-md" style=""><input class="form-control k-input-inner valid" required="" data-val="true" data-val-required="Enter the date observed." id="ADR_Date" name="ADR.Date" type="text" value="6/07/2023" data-role="datepicker" role="combobox" aria-expanded="false" aria-haspopup="grid" aria-controls="ADR_Date_dateview" autocomplete="off" aria-disabled="false" aria-readonly="false" aria-describedby="ADR_Date-error" aria-invalid="false"><button aria-label="select" tabindex="-1" class="k-input-button k-button k-icon-button k-button-md k-button-solid k-button-solid-base" type="button" role="button"><span class="k-icon k-i-calendar k-button-icon"></span></button></span>

from Custom Validation TagHelper: Modify Child Element