Go to a specialized store because in it you will find greater variety of products and greater experience of their sellers. In addition to direct service and many times free of charge.
It is important to take your child at the time of purchase it, to choose the appropriate size for the.
Choose the size appropriate for him, or any that are to be adapted, the most simple possible (18 speeds for a toddler makes sense, shock absorbers not as mechanically complicated operation in addition to adding weight, making the use of the same).
Check that your child can easily operate the brakes.
Ask for the after sales services.
Buy a helmet to your needs and teach you to use it.
To buy a used bike
Pay attention to some details, and follow the same advice for a new bike.
Check the State of the seat, which is not loose or severely damaged on the surface.
Check the condition of the tyres which are not cracked, wheels with spokes tight and focused, chain without rust.
Watch the State of the paint and oxide, since that will tell you possible cracks in the frame.