moving an object in a circular motion based on the parent

I am trying to make an object move in a circular motion on an xand y play as it is a 2D game, but how would I do that: I have this

transform.Translate(0,0,Time.deltaTime*0.1); // move forward
transform.Rotate(0,Time.deltaTime*0.1,0); // turn a little

This works but I want to change the speed of it as well for differnet levels of my game so that is harder. e.g. at first it is slow but then in another scene it is faster than the first one. If anyone could help that would be great. Thanks in advance!!!

this is what i figured out in the end

using UnityEngine;
using System.Collections;

public class circularMotion : MonoBehaviour {

	public float RotateSpeed;
	public float Radius;

	public Vector2 centre;
	public float angle;

	private void Start()
		centre = transform.position;

	private void Update()

		angle += RotateSpeed * Time.deltaTime;

		var offset = new Vector2(Mathf.Sin(angle), Mathf.Cos(angle)) * Radius;
		transform.position = centre + offset;