Hi. We've been riding around with bike flags as a family for more than a year and a half now. We've been riding in all seasons (in Northern California, so there isn't a ton of variation) and in rain and in fair weather. I wanted to share our experience of riding with a bike flag for kids.
Pros: Why should I ride with a flag for my bike?
Bike flags make the rider more visible
The main reason to use a flag is visibility. Until we get more protected bike lanes, we often need to share the road with cars, and it's important to be seen by drivers when on the road. The flag poles typically mount to the axle of the rear wheel of the bike. While riding to school, we can see a bike flag from 3-4 blocks away. They're visible because they're so high off the ground, and the dynamic motion catches your attention. Childrens' bikes are low to the ground and riding with a bike flag can especially be beneficial for kids, especially in conjunction with childrens bike lights.
Being visible and predictable are the two biggest components of bike safety when sharing the road with cars, so the safety flags can help a lot.
Special case: Bike Trailers
Bike flags are especially helpful for bike trailers. If you're towing a kid behind you, whether it be in an enclosed trailer like a Burley or with a trail-a-bike type setup, they may be only a few feet off of the ground. Having a tall flag will help make sure others see your bike trailer. Our bike trailer converted into a stroller (We had a Thule Chariot), so we were able to use the bike flag as a stroller flag as well.
Bike flags are fun
You can put your own message on a bike flag pole. Or the flag itself can be cute, colorful and fun. I'm hoping to make a custom "Bike to school" flag for riding to school next year. There's a new flag every year for Portland's Pedalpalooza summer series of rides.
They are even more fun if they light up at night. For day and night time use, I've built a bike flag that lights up in different colors to make a ride more fun while keeping the rider visible.
Cons: Reasons to not put a safety flag on your bike
Though bike safety flags can help most people, they aren't for everyone. Here are some reasons why a bike flag might work out.
Height: Bike flag poles that are too tall
If the flag is too tall, it can be hard to get on and off of the bike. Many people swing a leg over the saddle when getting on their bike, and a very tall flag may make this difficult. A tall bike safety flag pole can also snag on objects that you ride under, or they can snag while you walk the bike around. Most flags can sort of bend out of the way as necessary, but this can still be annoying.
Weight: Bike flag poles can make a bike feel more tipsy, especially for small bikes
When prototyping our bike flag light, I learned that a heavy bike flag pole can make it hard for a kid to ride. I put a 6' long flag pole on my child's 16" bike. They complained that it was "too tippy" to ride. The flag swayed side to side a ton, especially because they were still younger and not as capable as riding in a straight line. As a result, I made the flag pole shorter.
I hope you've found this bit of information helpful. I'm happy to chat more and share my experience, just leave a comment!