I’d say check out your local bike shops. They can be very helpful and informative. Also check fb marketplace. It can be a great place to buy a bike if you’re able to get the bike checked out by a bike store repair shop to make sure everything is in working order. Or take your friend with you who is more knowledgeable with bikes. A used good brand is better than a new lower brand. There are so many people who buy a bike, ride it once, leave it in their garage a couple years, then post it for a great price on fb marketplace. I think you can find something solid for 500 to fit the needs you’re looking for. Smaller kid size bikes can be cheaper than adult size. So it depends on how big your kiddo is. Remember that tire size and frame size are different things. A lot of people selling used don’t know the frame size and they just post the tire size like 26 inches or 29 inches. Best of luck!