Tell them to read this.
Blockland, when paid for, has a limit of 256,000 bricks. The BIGGEST LEGO set, the Taj Mahal, has no more than 5922 pieces. You could build almost 43 of them in Blockland!
A typical LEGO brick is 1 cm tall. For Americans, that'd be about 0.4 inches. So, 256000 bricks stacked up could reach 100787.4 inches, which is 8400 feet, which is a little more than one and a half mile.
In addition, Blockland's $20 dollar fee is one-time, just like if you bought your son a video game.
Blockland also has hundreds of add-ons, ranging from virtual cities where you can make a living selling things, underground mines which go on forever (and I really do mean forever), horses and cows you can ride, and even a jeep that goes up walls!
You can even make music, shirts, and faces to put inside the game. There are always lots of people playing something, so there's never a boring moment in Blockland.