Recreational & Family - Help with frame for 5 yer old!

My son rides a POS transformer bike that is holding him backfor any kind of distane it feels like it weighes as much as my GT MTB. I woul like to get him a BMX 20" bike but the frame will need to be very small. Any help will be appreciated. I do not mind building the bike or piecing one together but I need help where to start. It might be a used bike or new I have no idea.

Visit a LBS that sells used bikes, craigslist, and e-bay.
I found a used trek jet for $60 in good condition, far lighter then walmart/target jobs and a bit cheaper.
The seatpost is about 4-5" above the seat tube, should fit an average 5 y/o standover wise, the top tube might be a little long.

Check this out. I got one for my kid. It's a 16" BMX bike. I switched out the crankset so the pedals wouldn't hit the ground while cornering.

http://i230.photobucket.com/albums/ee277/prenex/P1030133.jpg

The Lil Ripper is awsome!!! I would get him a 16" again but he basically needs more gear and a lighter bike. The Transformer bike his Aunt got him is killing him but he still goes 4+ miles but he pedals so fast and if he stops pedaling its like an auto brake is ingaged!

I just got a used Kona Makena for my 5 year old, it is about 10 pounds lighter than his previous "bike". I was looking for a trek or specialized bike for him. All of these have aluminum frames and reasonable components.

If he's too big for a 16", all the big makers (Cannondale, Trek, Specialized, Giant, etc.) make a 20" kids MTB.

Be careful with the BMX - you need to make sure it's a kids' bike. There are a lot of 20" adult BMX bikes out there that are short enough but way too long.