You'll probably discover that "substitute" computers for your kid to bang on are no substitute (in the kid's head) for banging on the real thing. At least, that's our observation with our kid.
That said, there are plenty of kid toys, some of which look explicitly like laptops, while costing only $50 and being safely smashable. Unfortunately, they also tend to be loud and obnoxious.
Despite all that, you may find that some of the Flash games put online by your favorite kids' TV shows can be quite engaging. Sesame Street's Flash stuff is remarkably comprehensive.