var showData = document.getElementById("showData");
var button1 = document.getElementById("but1");

var usersRef =
firebase.database().ref('stores/').orderByChild("sid").equalTo(123);



function s2_but() {   //function gets trigger when button pressed
    usersRef.on('value', snap);

function snap(data) {
    data2 = data.val();
    console.log(data2);
    showData.innerHTML = data2.sname; //sname is the name of child key
                                      //whose value I want to show
       }
};

entry1: {prod1: "coffee", prod2: "sandwich", sid: 123,
         sname: "Java Coffee"}
__proto__:Object

<p id="showData"></p>

showData.innerHTML = data2.sname;