Re: A bicycle engine kit for a 11 year old? YES or NO
It would be great for you and your son to build a bike together for him.
If i was you, you will want to govern his speed and ride together with him off road at a safe place,thats if you want him to live long enough to enjoy it. You will also want to go over the safety stuff about the bike and power tools too,if you want him to keep all his fingers.
Ride safe and enjoy.