Honestly, I'm disillusioned with the ras pi. Its GUI is shit. Its command line interface is fine, but your kid needs to be pretty damn persistent to work with the CLI at a young age. It's also pretty bad at interfacing with hardware - you need a lot in between the GPIO (connections used to output/input signals) and your eventual hardware. (Buffers, controllers, level shifters, overvoltage protection, and so on.) By the way, I own three ras pis.
The arduino on the other hand I would gladly recommend to anyone who wants to learn programming and hardware. Get a starter kit ($50-100 depending on how much you want to spend) and you can control motors, send signals wirelessly, wire up various basic input/output devices, and so on. Want a wifi- or bluetooth-controlled car? Yeah, you can build that for about $100 with an arduino.
By the way, the arduino really set me on my current path. I knew how to program, and figured I wanted to go into electrical engineering to "round it out," but the arduino really showed me that I enjoy the whole EECE (electrical and computer engineering) stack, top to bottom. Something like $100 taught me what a university charges about $10k for. Or $100k, depending. I am by the way almost entirely serious - about $500 in hardware, and a lot of effort, will teach you almost an entire engineering curriculum. (By "a lot of effort" I mean "several years of dedicated study, thinking, and several hours daily average." Otherwise known as approximately what a university curriculum requires. I recommend doing both in parallel like I did.)