Maggie, one of my kids had that problem at about 5 or 6. Refused to eat any fruit or veg or anything with fiber.

WARNING: If you are squeamish look away NOW!

Started projectile vomiting bile. Xrays showed extreme constipation.

I had to start disguising fruit and veg. Told her carrot cake was orange coconut. Pureed pulses and whole grains in hamburgers, etc.