steps:

appBar: AppBar(  

            centerTitle: true,

            title: Text('HolidayLandmark'),  

            bottom: TabBar(  

              tabs: [  

                Tab(text:"Trips"),

                Tab(text:"Events"),

              ],  

            ),  

          ),  

          body: TabBarView(  

            children: [  

              TripBooking(),  // from TripBooking.dart file 
              EventBooking(),  // from EventBooking.dart file

            ],  

          ),

TripBooking.dart

appBar: AppBar(  
            toolbarHeight: 0,
            bottom: TabBar(  
              tabs: [  
                Tab(text:"MyTripbooking"),
                Tab(text:"ManageTripsbooking"),
              ],  
            ),  
          ),  
          body: TabBarView(  
            children: [  
              MyTripbooking(),  
              ManageTripsbooking(),  
            ],  
          ), 

MyTripbooking.dart

  child: SingleChildScrollView (
         child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              SingleChildScrollView(
                   child: Center(
                     child: Container(  
               child: new ElevatedButton(
                       style: ElevatedButton.styleFrom( 
                       primary: Colors.blue,
                        ),
                          child: Text(
                            'MyTripbooking',
                            style: TextStyle(
                              fontFamily: "Roboto",
                              color: Colors.white,
                            ),
                          ),
                          onPressed: () {
                          })
                 ),
                   ),   
                ),
            ],
          ),
        ),

ManageTripsbooking.dart

         child: SingleChildScrollView (
         child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              SingleChildScrollView(
                   child: Center(
                     child: Container(  
               child: new ElevatedButton(
                       style: ElevatedButton.styleFrom( 
                       primary: Colors.blue,
                        ),
                          child: Text(
                            'Search',
                            style: TextStyle(
                              fontFamily: "Roboto",
                              color: Colors.white,
                            ),
                          ),
                          onPressed: () {
                          })
                 ),
                   ),   
                ),
            ],
          ),
        ),
    );

Output