I'm an engineer, and I am a parent of a 4 year old (soon to be 5) and one that's about to be 2. The 2 year old is really comfortable with 'Duplo' (the double size lego sets) as he doesn't have the coordination to manage small parts yet, and there's a choke hazard. The other thing that fits the 2 year old well is the wooden Thomas the train sets. We spend hours building elaborate track sets.
The 4 year old, on the other hand, is helping me to build a wooden spaceship for part of his space themed birthday party in July. He has his own hand tools, but I've also gotten him an electric screwdriver and a small set of hex base drill bits which he uses only under my supervision. It's really fun for me when he specifies design changes with the reason 'After all, daddy, it IS my spaceship.' The tricky part is keeping things moving fast enough to maintain his interest, but still promoting tenacity towards the goal of completing the project and making it cool.
I don't necessarily want to turn my sons into engineers. That would be fine, but I really want them to experience the value of concepts like 'work vs. reward', 'make your own fun', and especially 'turning your own imagination into reality'. No matter what they do, these concepts will take them somewhere.
One thing I've learned is that my kids each have a compass needle. They gravitate towards natural abilities and interests, and I want to provide opportunities in those areas, but I also have to bolster the areas they don't naturally excel in.