Each game has a strict time limit, and takes place on a fairly small map. Unlike most online games, where it's often a case of every man for themselves, people tend to work together a lot more on World of Tanks, almost silently dividing themselves up into groups. With only one life, and one tank to use, if you get blown up, it's game over to you, and you're sent back to the main menu - so being cautious, and working together as a team is essential. If you're finding things hard, you can always join up with a group of friends in an online platoon, which makes the teamwork element a lot easier - or, you can have a go at the training ground, where you can face off against computer controlled opponents as you learn the ropes (and gain XP and silver coins as you do so)
In terms of accessibility, World of Tanks can be pretty challenging to grasp when you're first setting out. Starting off with just a handful of tanks, you face something of an uphill struggle until you've managed to buy your first few upgrades. Spending your first few games exploding almost as soon as you spot an enemy tank may put some children off, but those who stick with it will find the game goes downhill from thereon in. As a helpful note, if your child's really struggling, it's probably worth taking a look at the Proving Grounds mode, where they can earn XP and silve coins for their tanks (which let them buy upgrades) by taking on easier computer controlled opponents.
There's a lot of strategy, and different elements for kids to think about here, though - some tanks simply don't have the firepower to do damage to certain tanks; while others can damage them, but only if they hit a specific part of the tank, or shoot from a certain angle. With several different types of tanks on offer, each with their own strengths and weaknesses - and little room to make mistakes - it can be a pretty steep learning curve to begin with.
As a free to play game, there are several ways you can spend money to get ahead in World of Tanks, although you don't have to spend a single penny on any of it. You can unlock the vast majority of the tanks and upgrades; play together as a team; and play on all of the maps without spending any money at all. What money you do spend is simply used to unlock things quicker, as the gold coins you buy can be spent on purchasing upgrades, new tanks, or buying extra slots in your garage, so you don't have to sell your old tanks when you buy a new one.