Found the issues we were having

1. forgot to establish the dependencies for thecalculated value, hence it was never firing.

2. get child was not working, so we changed code to:

dojo.provide("com.ce.team.workitem.attribute.riskProbabilityCostProvider");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function() {
    dojo.declare("com.ce.team.workitem.attribute.riskProbabilityCostProvider", null, {

    getValue: function(attribute, workItem, configuration) {

        try {
            //console.log("Executing cost provider");
            var consequenceCost = parseInt(workItem.getValue("com.ibm.team.workitem.workItemType.risk.consequencecost"));
            //console.log("Cost = " + consequenceCost);
            var probabilityAttribute = workItem.getLabel("com.ibm.team.workitem.workItemType.risk.probability");
            //console.log("Prob Enum: " + probabilityAttribute);
            var probability = this.__getLastIntSegment(probabilityAttribute);
            //console.log("Probability: " + probability);
            var calcCost = (consequenceCost * probability) / 100;
            return calcCost.toString();
        }
        catch(err) {
            var txt;
            txt="There was an error on this page.\n\n";
            txt+="Error description: " + err.message + "\n\n";
            txt+="Click OK to continue.\n\n";
            console.log(txt);
        }

    }, //end getvalue

     __getLastIntSegment: function(identifier) {
      if (identifier != null) {
         var lastSeparator = identifier.lastIndexOf('%');
         //console.log("Sep index: " + lastSeparator);
         var numberString = identifier.substring(0,lastSeparator);
         //console.log("Number: " + numberString);
         return parseInt(numberString, 10);
      }
      return -1;
    },

    __sentinel: null

    });
})();

We now have it working, thanks for all the feedback.