Select Child Layers.jsx Photoshop Script - Selects the first level of layers inside the currently active group.

Select Child Layers.jsx

// https://gist.github.com/joonaspaakko/1add5c6a905216740c9c922a657636e1

#target photoshop
if ( app.activeDocument.activeLayer.typename == 'LayerSet' ) {
   app.activeDocument.suspendHistory( "Select Child Layers.jsx", "init();" );
}

function init() {
   
   var doc = app.activeDocument;
   var sourceGroup = app.activeDocument.activeLayer;
   // Make a temp group with a temp layer inside it:
   // [Parent group]
   // - [TEMP GROUP]
   // - - [TEMP LAYER]
   var tempGroup = sourceGroup.layerSets.add();
   tempGroup.name = 'TEMP GROUP';
   var tempLayer = tempGroup.artLayers.add();
   tempLayer.name = 'TEMP LAYER';
   // Loop through all direct child element and move the last layer below 'TEMP LAYER'
   var children_length = sourceGroup.layers.length - 1;
   for ( var i = 0; i < children_length; i++ ) {
      var lastLayer = sourceGroup.layers[ sourceGroup.layers.length-1 ];
      lastLayer.move( tempGroup.layers[0], ElementPlacement.PLACEAFTER );
   }
   
   // Get rid of TEMP LAYER
   tempLayer.remove();
   
   // Get rid of TEMP GROUP by ungrouping it, which also happens to select its children.
   doc.activeLayer = tempGroup;
   
   // Ungroup tempGroup
   // =======================================================
   var idungroupLayersEvent = stringIDToTypeID( "ungroupLayersEvent" );
       var desc702 = new ActionDescriptor();
       var idnull = charIDToTypeID( "null" );
           var ref427 = new ActionReference();
           var idLyr = charIDToTypeID( "Lyr " );
           var idOrdn = charIDToTypeID( "Ordn" );
           var idTrgt = charIDToTypeID( "Trgt" );
           ref427.putEnumerated( idLyr, idOrdn, idTrgt );
       desc702.putReference( idnull, ref427 );
   executeAction( idungroupLayersEvent, desc702, DialogModes.NO );

}