Are cheap Amazon kids helmets safe?

It depends on certification. Community reviewers flag that some no-name helmets sold online have been recalled for failing to protect in a crash, and that CPSC certification is only rated to about 15 mph. Helmets with documented expert testing behind them (the XJD toddler helmet and Retrospec Scout both appear in independent roundups) are safer bets than unbranded bulk listings with thin review histories.