There is no SUM operator in Salesforce. In order to sum child records in salesforce you need to use Rollup summary field for Master-Detail relationship Objects. If you want to sum up child records related through Lookup relationship, then the easiest way would be to use Lookup Rollup Summary 3rd party App: is direct install link if you are interested:

FYI this app will not work if using Professional Edition. Another option for rollup summary app if using PE is “Rollup Helper”: