A 12-inch or 20-inch basically depends on how tall the kid is — most buyers go with the 16-inch for that 4 to 5 sweet spot, and it's pretty much the same frame geometry across range with a steel tube (Φ22 by 0.8 to 1.5mm) that handles sidewalk bumps fine, just not off-road jumps.
You've got four brake types depending on teh build — caliper or V-brake for the front, band or foot brake for the rear, and the tires are either air-filled (1.75 to 2.4 inches) or EVA foam if you want to avoid flats, though the air ones ride smoother; the chain wheel runs 24 to 28 teeth with an 89 to 127mm crank, and the flywheel is 16, 18, or 20 teeth.
It's actually not great for kids who already ride confidently without training wheels — the steel frame and 4-point handlebar make it a bit heavy for that, and we usually have stock on most colors (red, blue, green, yellow, black) but the basket or flag are optional add-ons.
Welding is argon-arc or CO2 depending on the batch, saddle options are leather, spring, plastic, or PVC with the spring one being more comfortable for longer rides, and the rim can be steel alloy, aluminum, or plastic — alumnum is lighter but costs more, obviously.
Packaing is SKD or CKD in an export standdard carton, and the MOQ is typically 50 units or so for mixed sizes, though our company can do a smaller trial order if you confirm lead time first.