GW2 Heart of Thorns & Base Game
Heart of Thorns (HoT) is the first expansion pack released for Guild Wars 2 (GW2). It takes an increased gameplay learning curve from the base game - which was very user friendly, and forgiving, particularly for those who didn't have much time to dedicate to a game. The four maps that came with HoT, however, revolve around "meta-events" - meaning that you have to work with other players to complete smaller challenges around the map in order to work up to a final big event. It requires understanding of tactics, the ability to communicate, knowing your profession and of course, when to dodge. For that reason, it may not be so kind to the younger players, so you might want to keep any eye on them, or play alongside them, as the map chat can get abusive (you can also put the language filter on). You can select to block certain players so their messages cannot be seen, and you can report anything overtly abusive. ArenaNet is very good at dealing with such cases.
There's no bad language I can actually think of in the game itself, but there are some words used instead - but this is no different to saying something like "Oh, sugar!".
There's elements of platform gaming in GW2, and it's an fast-paced action combat system - you have to learn to dodge, and there are various places in the game in the first maps where you can do this.
Some aspects of the game might be scary for younger kids, but at the age of 12 and above it should be fine.
This is a buy-to-play game, meaning you'll pay a lot up front - these days, however, the base game and expansion are bundled together, so if you don't already own it, it's not too bad. You can also sign up as a free player for the base game, which is what I'd recommend for any potential new players anyway! Once you have the game, you don't need to pay again - there's no subscription fee. There is the Gem Store, which is the online store for the game. It's only cosmetic stuff in there, and you can use your in-game money to make purchases - although you will have to save up a bit before you have enough to do that. It could arguably be a good and safe way to teach your child how to save their money up for something they'd like. You can also use PayPal or a credit/debit card to buy gems up front - with an option to save your payment details. You can also gift items to other players - I have gifted my daughter some outfits and mini pets this way.
There are some decent role models in the game, and you will generally play on the "good" side, and be saving the world. You'll help out the game's citizens in myriad ways - one day, you'll feed their cows for them, or fix plumbing leaks. The next you'll be defending their village from monsters, putting out fires and then going back to do some gardening.
The expansion has also brought gliding to the game, and you can do this in both the core base game world, and the new maps. It's very fun, and I think most kids would really like it.
Lastly, as with most MMOs, there's some over-sexualisation, specifically of female humanoid characters and the armour/clothes they wear (charr - big beastial cats, and asura, tiny goblin-esque creatures don't have any clothing differences between genders). A light discussion with your child about this, and sexism in gaming, may be needed. There's also readily-available transmute crystals, which swap your clothing's appearance, for anything you feel is not appropriate. There are plenty of modest appearances available, some of which are actually quite cool. You can also change the graphic settings so the armour of other players displays as plain generic non-sexual armour.
It should be noted there is no other time in which gender plays a role in the game - female characters can outnumber male characters on some occasions, and depending on the player's choices. In this respect, it's a very positive, progressive game. It does have non-player character (NPC) romances, a few of which are same-sex, and these are portrayed in an equally positive light, as they should be: these are definitely characters who just happen to be gay, and can also provide a great point on which to talk to your child about same-sex relationships. Your own character cannot have a romance. Your own character cannot "die" in the traditional sense, either - they can only become unconscious to the point of which they look asleep.
In summary, I'd recommend this game for kids of 12+ (which is its age rating here in Europe anyway!), but younger kids can play in monitored situations. I'd recommend playing with your kid or sitting with them whilst they play.
The GW2 community is generally very friendly, but as there's always a few bad apples, I would not advise letting your children on the official forums or the Reddit threads to ask questions about the game! You should do that for them if necessary.