One of the most popular games like Poptropica is Club Penguin, a MMO RPG for kids which is massively popular.
Games like Club Penguin are about meeting and working with other younger gamers
You can play Club Penguin for free, though there are also paid memberships and some content that can only be unlocked by purchase.
As you might expect from a game called Club Penguin, you play as a penguin. As a penguin you will be able to explore an expansive game world. There are games to play and you can communicate with other players, forming a team and creating an igloo.
One of the best things about Club Penguin is the sheer amount of content on offer. It is definitely one of the biggest games for kids in the world.