This site is perfectly safe, and a great online game for your child (with profanity filters, a great environment, etc.). I will say that it DOES have an older audience than the old Club Penguin, so your child will likely be playing with a large portion of young adults (but as I mentioned before, there are systems in place to prevent significant trouble from occurring because of this).
Club Penguin Rewritten is just like the old Club Penguin. As far as I know, there aren't any ads on the site. However, many of the people using the website are teenagers, so if you let your child on, be cautious of that.