An excellent start your truck has a nice feel to it, not too babyish and not too much detail either. And nice use of contrasting wood's as well.
I made a tractor and trailer for my boy but the tires were smooth no tread's like yours. I was thinking possibly a way to do wheels could be to turn them (or bandsaw/sand them), and use a combination of a woodturner's indexing system to rotate the disc acurately for even spacing's with a router-and-jig to machine small but accurately spaced and lengthed "treads". You would keep the solid wheel then no need to fuss about lining them up?? Just a thought. I know next time I do a wheel I'll use the lathe so as to get dead centres, even a mil out turns the wheel into a cam :roll: :lol: :lol:
cheers Jonathan