Yama Jama 24"
It is not out of the ordinary to experience dropped chains when pedalling backwards when it comes to 11-speed cassettes, especially on brand-new bikes. If you Google this, there are over a million results for both kids' and adult bikes. Since the size of the cassette on the 24" is the same as on adult bikes and the chainstays are obviously smaller, the chain deflects at a higher angle.
The bike already comes with a narrow-wide chainring for better chain retention so if the shifting is set up correctly and the derailleur hanger is not bent, the tension and chain drop will improve greatly over time (but might still occur if pedalling backwards aggressively).
- 99% of the time chain drop is due to a bent derailleur hanger (young kids often drop the bike on the "expensive" side). Even a slight bend can cause issues. Get a replacement
-ensure shifting and derailleur are set up correctly
- make sure the rider is not pedalling aggressively/standing up on pedals while shifting