You can actually get whole computers which are designed for children.
They are usually much cheaper, and more sturdy than an ordinary computer. Generally they can only do a few things though.
I also once bought for a young cousin, this system that was designed to be used with an ordinary computer.
It had a keyboard that fitted over the usual keyboard but had bigger keys for children's hands and it loaded up reading and math computer software for young children.
It was a bit clunky so I don't know how much it was used, particularly once his parents got a tablet. Kids pick up how to use that kind of thing really fast and having a touch screen made it much easier for my cousin to do whatever he wanted with it (obviously under a lot of supervision!).