Usually 12" to 16" wheels. Look for single-speed models to keep things simple.
A heavy steel bike can weigh half as much as the child, making it hard to control. Aim for a lightweight aluminum frame if your budget allows.
Always pair the bike with a properly fitted helmet . Look for built-in safety features like chain guards (to protect fingers/clothes) and reflectors .
Avoid the temptation to "buy big" for them to grow into. A bike that’s too large is difficult to maneuver and can discourage a new rider from ever wanting to get back in the saddle.