12 inch balance bikes are fine, but 12 inch pedal bikes are a waste of money, in my opinion. This article has good reasons, why they are a waste, too. http://www.twowheelingtots.com/...d-hotrock-12-review/
We started our first child with a strider balance bike and then I picked up a air filled tire 12 inch diamondback balance bike (this was faster and better over rougher terrain), and then bought a 12 inch specialized hot rock. Don’t make that mistake, as I wish I read the article that I linked above prior to buying the 12 inch specialized. The diamondback was great for balance and confidence and both of my kids went straight to riding pedal bikes by their mid to late 3s. Training wheels do a disservice, in my opinion. I ended up buying a isla bike cnoc 16 inch when my older son turned 4 and 1/4. When my smaller son was about 3 3/4 he went from the 12 inch balance to the cnoc 14 inch (we sold the 12 inch specialized because it’s really heavy and a 12 inch pedal bike fits a child crappy. 6 months later he went to his older brother’s cnoc 16 inch, and the older boy just before he turned 6 went to a 20 inch isla with 7 gears. Isla bikes was founded by a racer and has designed the bike to fit small bodies, exceptionally well, which makes riding a joy for the little ones, just like a well fitting adult bike.
Our kids love cycling and we have a liberal bike budget for them, as the resale value of these bikes is really high. Google islabike reviews or see what they have sold for on eBay. The also make crazy high end kids bikes too, but those are a bit overkill, unless your little one becomes a big time racer. Our older boy did fine in a couple youth triathlons, I just exchanged out the wheels for 20 inch continental slick tires.
I should add they are really really light for kids bikes, too.
http://www.islabikes.com/starter-bikes/.]
I dont know about availability in US (I’m in Europe), but I’d seriously recommend Frog-bikes. They are light, and our 5 1/2y.o. has had his 14" for over a year now - not being able to break it (which is a miracle, if you saw how it gets treated!
We also bought a 12" balance-bike for our (then 2 yo. girl) - which was great (also super ligth, and easy to mount/dismount).
I’d also like to add that i think the kids should stick to balancing-bikes until they are ready to go without training-wheels. Our boy got his balancing-bike around 2y.o. at around 2.5 years he became a real fan, and would really get the hang of balancing (and he could go scary fast downhill!). For his 3rd birthday he got a 12" pedal-bike with training-wheels, but didnt really like it (because the traininer-wheels would limit his handling of the bike). Therefore, he stuck with the balance-bike a couple more months, and at around 3.5 years he moved on to the pedal-bike without trainer-wheels. First time without trainer-wheels he rode of on his own.
Our girl, on the other hand, has not been a huge fan of her balance-bike. She also has gotten her own 12" pedal-bike with wheels - and loves that one. I dont see her removing her training-wheels any time soon.
Long story short - get the kids on balance bikes, and keep them there until they are ready to pedal without trainer-wheels.