Tanker,
I hate to see anyone "scared off" by some of the information you might see posted here. The Chinese 2 stroke engines admittedly are not Hondas or BMWs. They are an inexpensive, fun way to motorize a bicycle. The level of mechanical skill displayed by the builder is an important factor in determining the success of the build. Yes, occasionally people get a lemon but the majority do not. You'll read far more horror stories than posts about the successful conversions. That's human nature.
You'll also see people telling you that you'll have to replace all of the fasteners/hardware. You'll read that you should disassemble the engine and do this and do that but there are thousands of successful bikes out there, built by mechanics and weekend novices alike that are running just fine with the kit supplied parts. You'll be told that the sprocket adapter method, commonly called the 'rag joint' is junk and won't work. BS. Installed correctly the rag joint/spoke sandwich will serve you well and in spite of what you'll hear, does not damage your spokes or cause them to loosen or bend.
What we see all too often is a rush to push aftermarket parts and accessories on the new builder by guys with limited mechanical abilities and want 'goof-proof' parts to bridge the gap between their skills and what is needed for a successful bike.
Do some more reading, explore your options as far as vendors are concerned and look for a reputable seller who offers a warranty and someone you can contact if you do have a problem. We always suggest to shy away from the cheap ebay sellers who might, or might not be around next week, or when you need them.
All of the sponsors you see down both sides of this page have good reputations for customer service. Choose wisely, take your time with the installation and feel free to come here for help or advice on your project.
Concentrate on the basics for your first bike. Stay away from so called 'performance' parts to make you go faster. Keep your first build simple and you'll be better off. If you're like most of us, you'll be building another one soon, then you can tinker and play with the hot rod stuff.
Enjoy this with your boy. You won't regret it.
Tom