Wrong forum. There are good Aussie dirt bike forums out there.

I used to be into off road bikes and probably what applies 20 years ago applies today, kwaka, Honda, Yama and Suzi.

I know technically this is the wrong forum but all sorts of stuff is discussed here, plus I know what it is like when some newbie jumps on a forum and asks for help straight up, look what happened to that "chick" that was looking for relationship advice on here, really constructive

thanks jam, very helpful

I'll third that. Stay away from the Chinese bikes. You'll spend more time repairing them than riding and the kids will loose interest. They are also so out of balance that they are dangerous.

As for size, see if anyone you know will let the kids try their bikes. It will depend on the child as to how big the bike needs to be. My son has been riding quads since he was 3 1/2. At 7 he had no hesitation of trying to ride mine (KTM 525). He then decided that he wanted to go to 2 wheelers and I bought a CRF80 for him. He wouldn't ride it as the size intimidated him. Bought a Yamaha pee wee 90 and he wouldn't get off it.
On the other side of the coin at 9 I would ride my fathers RM125 every chance I got. Dad had to hold the bike for me to start and catch me when I came in.

Go the 4 stroke if you can. Much more rider friendly and also quieter.
A semi auto bike will be easier to learn on than one with a clutch as well.
Buy a brand name and re sale will be a lot better.
Have a look around local and see what brands your local bike shop supports for parts.

Basic common sense with the condition of the bike. Look for splits in the paint on the frame which will indicate cracks, condition of sprockets and chain tension, ease of starting, colour of oil if a four stroke, state of air filter and also look at how much paint wear is on the engine cases. This will indicate how much use the bike has had.

Another thing to consider is their safety gear. As a minimum they'll need boots, helmet, goggles, gloves, neck roll and some sort of chest protection.
It will be quite expensive to set up for 3 bikes from the start.

Contact your local gymkhana club and spend a day or 2 there. They will be very helpful in getting you started and there will always be good 2nd hand kids gear for sale there as they grow out of it so quick.

i agree- go for 4 stroke chook chasers: less maintenance & more reliable in my opinion. i would add to go for the same brand for all 3, you will get to know a brand well and its quirks, and be able to work on them much more efficiently. i recommend honda, but that's just a personal bias- i reckon they are the best balance of reliability performance and price...

Do you want to race MX or just trail ride?

This will determine which bike as well.

I would say if your considering it seriously maybe even a few trips to local dealers. If you find one that seems keen to support the kids racing (not sponsorship) in any form that is buying factor as well. I know when you become part of the regular clientele and race circuit at many places you will get a lot of free help on race days, good advice and more genuine pricing in the long term.

Like John Hughes says. "chose your dealer before you buy your car bike"