function execOperation() {
    var context = SP.ClientContext.get_current();
    var session = SP.Taxonomy.TaxonomySession.getTaxonomySession(context);
    var termStore = session.getDefaultSiteCollectionTermStore();

    var parentTermId = '341ecc39-9cf7-4112-a183-faf4bef2d3f8'; //parent Term Id
    console.log(parentTermId)
    var parentTerm = termStore.getTerm(parentTermId);
    var terms = parentTerm.get_terms();  //load child Terms


    context.load(terms);
    context.executeQueryAsync(
        function(){
        //print child Terms
        for(var i = 0; i < terms.get_count();i++){
            var term = terms.getItemAtIndex(i);
            console.log(term.get_name());
        }

        },
        function(sender,args){
        console.log(args.get_message());
        }
    );

}