My kids and I ride six miles most days. My 9 year old is short for her age and rides a bike with 20" wheels. (I have a ladies bike with a 13" frame just waiting for her to grow into it, but she's about 6" too short). We let her set the pace for all of us. There are some days when she is fine riding, and other days when she will just start crying while she rides. If I ask if she's tired or wants to go home (we ride a large circle through the neighborhoods, but there are always ways to get home faster), she says no. She can't explain why she's crying, nothing hurts and she's not having trouble breathing. I thought it might be related to the heat, but today was a perfect day for riding and it happened again.
I don't even know how to ask her doctor about this, because it will sound like I'm somehow pushing her too hard. Is this something other kids do, ever?