If you really want to give your kid a nice bike, buy him a mini BMX racing bike...the kind made for little kiddie BMX racing. They are kinda pricey, but you can find them on ebay relatively cheap. Mini BMX bikes are without question the best bikes for little kids who have cycling parents who want their kids to do more than ride around the block. My son is now five but learned to balance when he was four. As soon as he would fit it, I bought him a Redline micromini BMX bike with 18" wheels. Like this....



It weighs less than 12 pounds (you can find them much lighter!) and has narrow rims that use high pressure tires, similar to road tires. They roll so well that you can gear it up high enough to really cruise. I've taken my son on 15 mile rides and we cruise near 10mph most of the way.

You could easily convert such a bike into a fixie by rotafixing a cog onto a BMX hub. A 5 year old isn't strong enough to unscrew the cog so a rotafixed suicide hub setup would be fine.

HOWVER, I STRONGLY RECOMMEND AGAINST forcing a 4-5 year old to ride fixed. It may be fine for a tricycle, which is much much simpler to operate, does not require balance, and is less likely to achieve the speeds that bicycles achieve. A bicycle, on the other hand, requires balance and has a potentially dangerous drivetrain. Little kids lack the coordination to ride with foot retention, which is an important safety feature on a fixie. Moreover, kids are not strong enough to pedal a high gear ratio and consequently must ride a much lower gear than an adult. Because of this, there is the potential to spin wildly out of control, sans foot retention (imagine feet and pedals flailing wildly), riding down the shallowest of hills and given the tendency for kids to crash frequently, there is a high possibility of getting hands or feet caught in the spinning drivetrain. Keep if safe, keep it freewheel, at least for a few more years. Don't rush it....focus on your kid learning to ride well. He'll easily adjust to a fixed gear DT when he is a little older.