I'll second the specialized kids bikes. They will last and most importantly they are way lighter than a Walmart bike. Your kid will be able to ride it much better than a cheap bike. This will result in them wanting to ride it more.

I bought the 20" ($250ish)for my 7 year old a few years ago. He moved up to the 24" ($380) when he was ten. The 20" was passed to my second son and will be passed on to my daughter, it is still in perfect shape. Unfortunately the 24" Specialized was stolen, we replaced it with a 24" Cannondale mountain bike which is also great, but slightly more expensive ($450ish).

I also highly recommend the strider for young kids (3 to 4 year olds). It has no pedals and they really learn how to balance. I used this with my second son. He rode that thing for about six months. Then one day I put him on a bike with no training wheels, gave him a quick push and he was riding fine on his first attempt.