how can i make this divider just below

You can use the widget Divider(height: 1)

you have to follow the given layout please refer and change your Layout then you will get the Divider under the Tabs

Column(
  // crossAxisAlignment: CrossAxisAlignment.stretch,
  children: <Widget>[
    Container(
        child: TabBar(
            controller: tabController,
            tabs: tabChildList,
            indicatorSize: TabBarIndicatorSize.tab),
      ),
    ),
    const Divider(
      height: 1,
      thickness: 1,
      color: ThemeColor.disabledColor,
    ),
    Expanded(
      child: TabBarView(
        controller: tabController,
        children: tabBarViewChildList,
      ),
    ),
  ],
);