For up to about 8 years old an Apple II is great. Lots of software free to
download, and almost indistructible.
Any older PC or mac for ages 7 to 10, or for the first year of computers at
any age.
Once they are through the basics though, figure on a fairly new system with
a good graphics board. Run all the older software in emulation, plus surf
the web and play games with decent speed.
Any house without at least 5 networked computers and two printers is fairly
deprived, my goal is about a dozen of different era's all talking etc.