input_page.dart

import 'package:bmi_calculator/gender_card.dart';

class InputPage extends StatelessWidget {
  Widget _buildCards(BuildContext context) {
    return ...
            child: Column(
              children: <Widget>[
                Expanded(child: GenderCard()),
                Expanded(child: _tempCard("Weight")),
              ],
            ),
          ...
  }
  ...
}