we started 2 of the grandkids out on balance bikes and the transition to pedaling was... well.. amazing!! the hardest part of learning to ride a bike is learning to balance. training wheels delay that process.. atbman has it right.
the oldest of the 2 started his pedal bike experience on an electra 16" with coaster brakes. this boy is on the slender side. and i did not know how much the weight of the bike can impact a childs experience riding... this bike had to weigh more than half of what he did. so last summer, i bought both of these boys frog bikes . through a bike shop that had a type of a buy back program. the 8 year old chose a bike with gears. and it weighs a lot less than the electra bike. we can't keep him off of it.. he has been out on hilly rides with me.. he says it is like magic how the bike goes up hill. the younger one at 4, after being on a balance bike for a year, took half an hour to get pedaling totally independently.. he could balance and pedal from the get go after a little shove off.. it took him half an hour to figure out how to get the bike started by himself.
there are many companies now putting out lighter weight bikes with better geometry and features now.. i think isla bikes started it and they are the gold standard. other companies that are out there are cleary (sausalito, california) and woom... woom bikes are readily available on amazaon. these bikes are very well made and can be found on sites like craigslist, however, they sell pretty quickly. google two wheeling tots for in depth information on how to buy a kids bike. isla roundtree has a very good video on you tube on how to teach a child to ride a bike (i can't post urls yet....)