My son turned 12 last May. He started out years ago with the typical little kid's bike. Everything went well until we took the training wheels off. He fell twice that day (he was 6) and refused to ever get on the bike again. Consequently, last year I bought him a KMX for his birthday. My husband was dead set against it: "too expensive", "he won't like it", "other kids will make fun of him", "he'll never learn to ride a real bike". In spite of the negativity, I bought it anyway. He LOVES it. Now, he rides frequently with me (I bought my trike so I could ride more easily with him), and we can all finally ride together as a family, without someone staying home. It WAS expensive compared to Walmart, but worth every penny. All the neighborhood kids think its really cool (and he loves the attention he gets from both kids and adults). As far as riding a "real" bike, well, my husband could be right about that one but I don't really care. I plan on buying him a nicer trike as soon as I can afford one but, in the meantime, we'll just keep riding and having fun. We have a regular 14 mile loop that we all enjoy and hope to expand upon this summer. My only regret is that I didn't find the KMX trike sooner.
My 12 year old came to bents this last year. She has been a natural on a bike since she was 6. At 9, I got her onto an extra small framed hybrid and by 10 she was doing 50 mile rides with us. On those rides she started complaining of her rear end getting a little sore. I let her try my bent and she took right to it. When she test rode a Rans Tailwind that a shop had left over, she took off without a wobble and really liked it. I wasn't able to afford a $1000 bike for her (we were getting her mom one actually) but I found an older used one for $350 and she rode it 40 miles a week later. It is now her main bike and she likes the attention she gets with it as well.
Lent a 14 year old my 2003 WW 3.4 trike for a 25 mile ride this weekend while I rode my GS GTO. We plan to do a half century early in May. He had a great time and would really like to have it permanently. Fat chance! Seriously, if you make it fun enough they will take every chance to ride.
I took my two sons (ages 9 & 11) to a bent event in The Netherlands. There were 8 or so different producers who put out loads of models, but only a few were adjusted small enough for kids. Everyone could just grab a bike and give it a test ride around the rollerblade track. I made them start out on the trikes to get the feel of lying back while riding. I then took them on a tandem bent with me. I was about to help them through the difficult challenge of getting used to a bent bike, when they just took off and left me in the dust. Sooooooo, no problem. The sales rep said kids usually just take off like that. It's the old farts like me that wobble a lot. Fine.
In September, my oldest son will have to start commuting to school (8 miles each way). He may want to get a bent to help him cut through the strong winds here. My younger son is more interested in dare-devil maneuvers. So, while he is proud of being able to master a bent right away, he will probably stay with a BMX for now. I think if we do get bents for the kids, we will look for second hand bikes to keep the costs down.
My son wanted to ride a bent "like Dad's" starting around age 6. At 8 he was big enough to fit on my wife's Rans Stratus (shown below) and I found a used Rans Tailwind. At 10 I bought him a used Burley Koosah. He still rides his Trek Mtn. bike occasionally, but complains of the butt/etc. pain. For longer rides he always chooses his bent.
My kids both have trikes, but they like to ride too many crazy places that a trike would just get pounded on, so when they are with their firneds, they use their upright bikes. When they get a little old and decide to stay on roads and trails more, I'll make them some two wheelers.