bbmike

His bike came with 54cm wide handlebar which is what adult bikes use. I adjusted the grips 3cm inwards which seems to be ok (48cm/19"), but it still looks too wide.

What width is recommended for a 4yr old Kid that is just learning to cycle (upright position)? His armpit measurement is (22cm/8.5")

fietsbob

Converse with your LBS, as they can see the situation.. drop by..

find a cheap bar and cut it down..
see what feels right by talking with your young rider,

kids seem pretty adaptable, and soon they will grow into
things seeming too big now..

bbmike

Thanks for your replay. I'm interested in opinions of what it should be or what worked for others. I already purchased lock-on grips so I can adjust their position however I want. The problem with kids is that they don't know what's good for them and their feedback is suspect.

I figure that for young kids strength is the issue so the position has to be, for one, so they can engage their largest muscles. Second, I asked him to run on his hands to see how wide he keeps them naturally while supporting his body. It comes to about 16" - so this is his next grip position...