Fetch Accounts by Parent Account

I am using Zoho C# SDK.

I want to fetch all child accounts of a Parent Account.

I am using this code snippet to get all child accounts-

"BulkAPIResponse<ZCRMRecord> response = module.SearchByCriteria("((Parent_Account:equals:" + accountName + ")", methodsParams);"

Its giving Error -
"ZCRMSDK.CRM.Library.CRMException.ZCRMException: 'invalid query formed' ".

Its NOT working for "Parent_Account".

Please give solution to fetch all child Acounts by Parent Acount.

Thanks