How do I add both onPressed and onLongPress to a Flutter Center Widget inside a Row?

I am trying in vain to add onLongPress to an image button along with onPressed. I get error: The named parameter 'onDoubleTap' isn't defined.

I have multiple rows of 2 horizontal image buttons using rows and expanded. Everything works except onLongPress (or onDoubleTap). What am I doing wrong, do I have to rebuild the entire code in a different format, or am I overcomplicating this?

I also tried to add a new child (error: already defined), GestureDetector, InkWell, with no success.

body: SingleChildScrollView(
      child: Container(
        child: Column(
          children: <Widget>[
            Center(
                child: Row(
                    mainAxisAlignment: MainAxisAlignment.center,
                    crossAxisAlignment: CrossAxisAlignment.center,
                    children: <Widget>[
                  Expanded(
                      child: FlatButton(               
                          onPressed: () {
                            setState(() {
                              launchURL();
                            });
                          },  

                          //Trying to add onLongPress , error: "onLongPress not defined
                          //If I try add a new child it says child already defined

                          onLongPress: () => _showAlertMessage(context, "message"),


                          padding: EdgeInsets.all(6.0),
                          child: Image.asset(
                            'images/image1.png',
                          ))),

                  Expanded(
                      child: FlatButton(                     
                          onPressed: () {
                            setState(() {
                              launchURL();
                            });
                          },
                          padding: EdgeInsets.all(6.0),
                          child: Image.asset(
                            'images/image2.png',
                          )
                      )//flat button
                  ),//expanded
                ])), //row-center

                //Repeat above for rows of 2 more image buttons

The code runs with a single onPressed for each button and does not display any errors, but adding any second click event displays errors.

Flatbutton only supports the onPressed callback, not onLongpressed, etc. Suggest you re-look at using gesture detector or other widgets that support long press. I would try using two containers, each wrapped in a gesture detector widget, as children in the row. Each container having a text and/or image content. You can then pick up onTap, double tap, long press, etc on each container. Not tested this as I am on my phone but should work. They are probably more elegant widgets to use than container.