1) What do the kids NEED the computer for?
2) What do the kids WANT a computer for?
In my opinion, it is never too soon to equip kids with a computer, provided you give them minimum assistance to avoid dangers (pedos, porn, etc.) and a games-only use.
A computer is nowadays such an important tool for ANYTHING, that I would really recommend you to get a propper modern PC for them. They will do their homework on them, do research on Wikipedia and Google, learn how to use Word, PowerPoint and Excel. Play games, get a social network with their friends, etc.
Also, bear in mind that, like it or not, Windows plays an important role in modern day IT infrastructure, one reason being the fact that many commercial applications are only available for Windows (i.e. professional CAD/CAM/CAE software) or simply are a standard.
Enforcing a Linux only policy on kids seems to me a similar thing as enforcing kids to be vegan (sorry if this sounds too harsh). And no, I have nothing against Linux and I do use it regularily.
You can buy a decent new PC for 300-500 Euro/US$, even a laptop.
A laptop is actually nice because it is cheaper than a PC + monitor and it can be used in the living room, the kitchen, etc. - wherever the parents are. Just explain to the kids that they are not allowed to take the laptop out of the house.
Raspberry Pi is nice, the RP3 runs pretty OKish now, but come on: this is not meant to be a full computer replacement. An old PC? Pfff... that sucks. To save you 200 Euro/US$ you are going to make your kids use old OS, old software and experience internet at unbearable slow speeds? Anything under/older than an Intel Core i3 just plain sucks.
Again, no offense, just my opinion.
Regards,
Vitor