I recently changed mine to a Microsoft Wireless laser keyboard 6000 v2.0 with optical mouse.
No problem todate and the keybord is easy on the wrist.

Some of the logitech keyboards and mice (mouses?) are quite good.
I've used the NewTouch™ Keyboard 200 - USB which is quite good. And my friends who game have that too. There is a gaming keyboard but I think that has programmable keys.
For the mouse - MX 518 Optical Gaming Mouse is pretty good. They also have G9 and G5 laser gaming mice but I haven't tried these. They also seem to have smaller mice for kids which you could consider.
http://www.logitech.com/index.cfm/keyboards/keyboards/&cl=sg,en#
http://www.logitech.com/index.cfm/mice_pointers/mice/&cl=sg,en#

I bought one of these mice
http://www.pcworld.co.uk/martprd/store/pcw_page.jsp?BV_SessionID=@@@@1740019606....

It's lovely to use nice and tactile and points where you point it.

As for keyboards, when we replaced the one that came with our Evesham PC, I bought one from a 'cheap' shop for £3 and it was by far the best keyboard I've used. We played WOW too and used this kbd for it.
Most computer shops have some nice kiddie mice.

I've got a Z-board for gaming - it allows you to insert different keyboard layouts into it for different games, but is perfectly adequate on the regular layout. Not sure if physically replacing the keyboard with something with a different physical layout counts as against the WoW rules, though (although how they'd detect it I don't know, and it strikes me as a rather silly rule, for what it's worth!)

http://www.amazon.co.uk/Ideazon-Zboard-Starter-Battlefield-Bundle/dp/B000MDXN0Y/...

As for a mouse (and also the keyboard!), you may want to get yourself 2 - one for gaming (a nice little precision laser optical mouse jobby) and a cheap and cheerful 2 button for the young folks to bash about.