For those of you who don't know, Nickelodeon did not create this show. In fact, Winx Club [the 'original' series] is over 10-years old. It came to America several times, being adopted first by the 4Kids network, briefly by Cartoon Network, and now by Nickelodeon partnering with it's studio of origin, Rainbow. -- Winx Club was created in Italy, it is a European cartoon.
As an illustrator, I actually like the artwork. Yes, the exaggerated proportions [specifically on the girls] make unattainable and unhealthy body image an issue that may need to be addressed with tweenage girls, but it's doubtful your five year old will worry about her figure after watching. Anyways, I digress; the style is very iconic and uniquely recognisable. There is no other like it that I've seen. When I see this artwork, I know it's Winx Club, where a lot of cartoons and animes get lost in trying to keep up with the hype and copy one another's aesthetics, Winx Club stands out. My son is 4 and has no interest, and my daughter is 11 and practically eats up my season 1 box set! Now there may be a lot of concerns by parents, but I assure you, overall, the show is fun, proves that good actions always win over bad, makes the good and bad very obvious and easy to understand, and praises positive actions, as well as stressing the ideas of friendship and family.
There are 6 seasons in the show, and each season is another year in time, so the main characters are approximately 20 to 23 years of age. They have had boyfriends since season 1 [most of them] and are in serious relationships with several girls hinting engagement. -- There is also the concept of arranged/forced marriage addressed by one member of the Club, Aisha who hails from a very traditionalist royal family. She is also around 17/18 years old at a time, so the young age may be problematic.
While there is no blood or gore present, there are still fights, most of which have been very watered down compared to the original series, where "actual" weapons have been seen used by "humans" [non-magical people on Earth] These are now omitted, and the characters who utilize magical powers turn to light swords, shields, power-blasts and dodging blows mid-flight instead. Despite the watered down fight scenes, there is a death in the show. The boyfriend/fiancee of the character Aisha is "killed off" at the end of season 4. His death is not graphic or violent, as the show is quoted, he absorbed "too much dark energy" to save his friends and dies.
Obviously, since this is a show aimed at children, there is no sex or hint to it in the show, but, I mentioned relationships earlier; the girls regularly hug and kiss their boyfriends, but there is no "graphic" or overly sexualised romance.
However, by from the pictures, you can tell that the clothing is quite revealing for girls supposedly in their teens [at the start of the series] As they age-up in later seasons, and enter adulthood/progress into their 20s, their outfits become subtly more mature and less revealing than the skimpy clothes they sport in their teens, so it's really a fading issue.
I put that learning really isn't existent in this show, because I believe it is mostly entertainment. There are some positive values present, but as far as having any educational value, there really isn't any. The show is purely grounded in fantasy and is meant [at least from my view] to be food for the imagination. It's wildly colorful, creative and fun. I would supervise younger children in watching the show for the first time or so, but for children 8 and up, I think they'd greatly enjoy it.