I have 2 sons aged 10 & 12…what should I get them to start?
yes, of course.
and now for an answer that includes the answer part:
I’d suggest a 20" or 24" (if they are taller) torker LX cycle:
http://www.unicycle.com/shopping/shopexd.asp?id=634
http://www.unicycle.com/shopping/shopexd.asp?id=675
They are inexpensive and will stand up to a moderate amount of use. They are also easy to learn on because they are lightweight and built with the correct crank/wheel ratio for control over speed.
double post oops.
I think that you misread the question. 
Yeah, the Torker LX are really great starter unicycles. From what I’ve heard that is, I don’t have one, but I’ve read that they are very nice. And if I were in the US, that is the first unicycle that I would have gotten.
The 20" would probably be better, the 24" is harder to mount, and a little harder to ride, but they could handle it. The 20" would probably better to start off with though, they later upgrade to the 24".
it also depends on what they are using the uni for
20 isn’t as efficient for travelling
The kids are 10 and 12 Get them the 20" lx you wont be sorry.
i have the 24" and wish i would have gotten the 20" so i would have it for freestyle after i got a trials and muni which im about to get.
The lx has held up really well i have done loads to it in the last couple of month that i have had it i can do up two stairs, drops of 3 ft and over 3 ft static side hop and i am 6ft 190lbs (1.8m 86 kilos) and the cranks have barley started to break i was very surprised becasue most of the people around here weigh less than i do and say that >1 ft drops bend it, not true i guess.
20" LX is what i learned with there really great unicycles
+1 
Just beware of the brick hard seat
I agree because it becomes very uncomfortable after a while.
I can disagree because the Miyata styles arent really that bad.
brick hard seat?
the LX seats are fairly comfortable. It isn’t a cx…
ok, now here is my input::
many people learn on a torker LX unicycle, it is a common unicycle to learn on. it will stand up to anything someone learning to RIDE a unicycle throws at it. if it is not used for tricks it will last forever. if tricks are being done, count on it being messed up eventually.
the other brands of learner unicycles aren’t bad nearly any aspect compared to the LX, but they lack the comfort of the seat and the handle that come on the LX
as far as size goes, they won’t fit on a 24" very well if at all, and you want some seat adjustability, so i would NOT get a 24". the 20" unicycle will do much better in this situation. (24" are better for older teens and up to learn on)
all this said, the CX is also a good unicycle, but the seat IS a “brick” , it is highly uncomfortable, but is also more replaceable than the LX seat.
+2
+1
LX’s are about the best learner uni there is. i would reccomend a 20".
Lx 20"
Yes - an 20" LX is perfect I am 10 years old and ride my 20" LX with the seat near the bottom, I would have more trouble on a 24" for sure!
Good Luck learning to ride