My son is now happy pedalling away
He started on a "Pukilino" indoor trike/scooter thing which isn't really a bike at all but it did get him used to steering round objects. It was only ever used indoors so it's mainly a handy mobile seat for him and footstool for us. He does still love it. I don't know if it was important to his developent as a cyclist or not, but he was using it pretty much as soon as he could walk (14 months ish).
Then a balance bike from Joe Joe Maman Bebe. Far too expensive, but I couldn't persuade SWMBO otherwise and he loved it as soon as he saw it, union jacks and all. He got that for his second birthday and was soon scooting along on it. An early downhill panic/crash did set him back a bit but it did teach him to respect speed and grass verges! He was happy freewheeling down hill with his feet up as long as it wasn't too steep (no brakes) but most happy scooting along on the flat at fast adult walking pace.
We tried a couple of goes on a trailer bike behind my mountain bike but to be honest I was never very comfortable with it. I couldn't see him but I could feel him leaning all over the place and making it very hard to steer while probably being very dangerous. We certainly never tried it in traffic. We might come back to this later on.
Now, he's 3 3/4 and his 4th birthday present was a CUDA bike in horrific Dayglow yellow and green with black and white "badger stripe" tyres. I've almost persuaded myself that it is Tour de Yorkshire colours... but no1 Son loves it to bits. He got it early and after a couple of sessions with a gentle grass slope and some careful instruction on brakes and pedals he was ready to try riding on a (very quiet) country lane. Last weekend he did a lap of York Racecourse on the cycle paths and coped really well with the extra traffic due to the Rally. He's happy uphill and down, and I'm absolutely chuffed to bits with him and his progress. We're seriously considering a trip to Dalby Forest on his fourth birthday (the family path, not the World Cup circuit
)
This was only possible due to the balance bike, they're an absolutely fantastic invention and I can't recommend them highly enough.
One piece of advice I'd give is that if you give a child a bike with V-brakes (or probably any other brakes) don't be tempted to adjust them to perfection in a moment of Proud-Dad efficiency. The first time we tried an emergency stop from 1mph he nearly went over the bars - I un-tweaked the front brake immediately so it gives mild decceleration at full grip. He can still lock the rear brake if he tries but I'd rather he slid the back than went over the front.