function _computeChildrenSideWidth( element ) {

	return element._children._boxes.reduce( ( accu, child ) => {

		// if ( child._bounds._needsProcess ) child._bounds.process( child );

		const margin = child._margin._value;
		const CHILD_SIZE = child._bounds._offsetWidth + margin.y + margin.w;

		return accu + CHILD_SIZE;

	}, 0 );

}
Return the sum of all this component's children width
@param {MeshUIBaseElement} element
@return {number}