gavgus,
After running a couple of bikes though the kids now my process is pretty much to get a bike of
eBay and then replace anything that needs it. If you have the time there are some real bargains that come up on
eBay, bikes that have been ridden twice and the kid is now to big, or dad bought the bike because he was a cyclist and the kid just isn't interested.
Pretty much everything (excluding some very nice Scott and a couple of others) have complete crap for running gear, so it will need to be replaced pretty quick anyway if your kids actually ride. Parts are also cheap from
Wiggle and the likes, as 8-speed is more than good enough though cranks can be very expensive to replace.
Also look for the DH/Dirt Jump style bikes, they have soft forks that actually work with the light body weight of kids and an alloy frame to keep the weight down.
My last purchase was a 24" Scott Voltage with cable disc's for $300 that was almost new from
eBay. $80 to upgrade to trigger shifters and derailleurs that actually work and it is a very good bike.
- David.