Create issue with cascading customfield child value using api

I have a powershell script that creates a new jira issue. i have a customfield associated with the issue that is a cascading dropdown. For the life of me i cannot get it to update the child of the custom field.

Here is the relevant part of my code

#Create Body of request
$ParentCreateIssueBody =
fields = @{
project = @{ key = $ProjectKey }
summary = $ParentSummary
description = $ParentDescription
issuetype = @{ name = $ParentIssueType }
assignee = @{ name = $ParentAssignee }
customfield_10250 = @{
value = “Process”
child = @{
value = “Audit”
} | ConvertTo-Json

#Create the jira issue
$ParentIssueCreation = Invoke-WebRequest -uri “$JiraURL/rest/api/2/issue/” -ContentType “application/json” -Method post -Body $ParentCreateIssueBody -WebSession $mysession -ErrorAction Stop | ConvertFrom-Json

if i remove
child = @{
value = “Audit”

the issue is created fine except of course the child field is not updated. The parent however is updated. yes Audit is a valid value and is under the Process parent. I can’t seem to get it to update the child and when i include the code for child i keep getting (400) Bad Request. please help.