Fun and Entertaining! But...
I've played this game for a couple of years now, and the game itself is great for younger kids (see below). It shows them how one part in a music piece may not sound all amazing alone, but as a whole they all sound wonderful. This game is pretty easy to function, without all the unneeded complicated stuff.
Players can friend other players, but it's totally safe. There is no private messaging, personal info shared, etc. This only lets people view your island (cannot edit it in anyway), like and dislike your islands (totally anonymous- you do not know who likes/dislikes them), and light your torches. You can breed monsters in the game (you select two different monsters which will create something new), and torches increase your chances of getting rarer monsters.
With all the friendly stuff, what makes this game not the first choice for younger ones? It's the in-app-purchases and how they convince you to buy them. When you breed monsters, you have to wait hours until they have finished (UP TO 60!), and then you have to go and out them in a nursery, doubling the time. Using a digital currency, diamonds, you have the ability to speed up that process. Coins are used to buy monsters, essential needs for your island, and decorations, and food is used to feed your monsters and transport monsters to better islands. There are also types of currency which are less used, such as keys, which are used to buy a specific species of monsters, and starpower (cannot be purchased), which is used to buy rare monsters and nicer decorations. All of this currency can be earned, but spent much more easily. Once players spend all of their currency and have to wait to earn more, they have a feeling of sadness as they really cannot do anything in the game for the time being. And that is where they go and buy currency, so the satisfied feeling doesn't go away. Currency can be very expensive, up to $100, and can be spent in one minute. Advertisements also greet you at the beginning of each play time, and can get very annoying. Of course, the only way to get rid of them is to purchase any kind of currency pack. you could easily disable in-app-purchases for young ones, but because the game is targeted around them it is most likely someone will turn them back on.
Overall, the idea of the game is great for young children, but if they are to play they should be monitored. The game is best to be played by responsible ones who will not go and spend all their money on digital currency.