Given the age and the rapidity with which the child will outgrow the bicycle, I wouldn't worry too much about replacement parts. I would purchase the lowest cost bicycle possible, including any shop costs to have it tuned and the wheel truing confirmed. Resale value, is rarely more than half the price of the bicycle, and a childs bike is not likely to be worth that much.
Despite the prejudices against department store bicycles on this forum, they are very appropriate choices for children (and many adults).
Any low cost bike is likely to need tune ups and probably wheel truing. If you don't have those skills they are valuable enough that it would be a good idea for you and your child to learn them as part of his new bike. Good source of information on the procedures and processes can be found in these sources
http://www.amazon.com/Master-Wheelbu...xgy_mov_text_z
http://www.amazon.com/Ready-Roll-Bil...f=pd_sim_mov_7
http://www.amazon.com/Park-Tool-BBB-...f=pd_sim_mov_6