The best way to get your kids into LEGO is to hand them a tub of LEGO bricks and say, "Here, go wild." But eventually, they may want to think even bigger and work with more bricks than they own. This free tool lets them build virtual LEGO models with about 10,000 different parts, and then purchase the bricks to assemble their creations in real life if they want to (and yes, they will want to).
LeoCAD, developed by Leonardo Zide, is computer-aided design software that's designed to allow new users to begin creating without spending too much time learning a new application. They start with a blank base and then search through the constantly updated parts library to find bricks they need.
Once they've selected a piece, they can choose its colour and then drag it into the building area, where they can rotate it and snap it into place. As they're building, they can see how their model is all coming together from different views:
It's a pretty straightforward setup with a basic tutorial to help get them started. Once they've mastered the basics, they can move onto larger models. Here's the Tower Bridge, an official LEGO set with more than 4000 pieces:
There are a couple of ways to purchase the necessary LEGO bricks after your builder creates the virtual model. You can search for them directly on the LEGO website, though that may take forever (see the 4000-piece tower). Or LeoCAD can export a XML file that you can import to the BrickLink LEGO marketplace, and you can start shopping from there.