How to create a child object of an object with a specific tag?

I would like the spheres that appear after pressing PLAY to be children objects of the object with the PLAYER tag.

How to do it?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class PathPlacer : MonoBehaviour {
    public float spacing = .1f;
    public float resolution = 1;
    void Start () {
        Vector2[] points = FindObjectOfType<PathCreator>().path.CalculateEvenlySpacedPoints(spacing, resolution);
        foreach (Vector2 p in points)
            GameObject g = GameObject.CreatePrimitive(PrimitiveType.Sphere);
            g.transform.position = p;
            g.transform.localScale = * spacing * .5f;

You can assign the parent object of the newly created GameObject:

g.transform.parent = someObject.transform;

You can get object by tag using GameObject.FindWithTag.