When sourcing children's costumes, the most critical factor is compliance with safety regulations. For the US market, products must meet CPSIA (Consumer Product Safety Improvement Act) standards, specifically regarding lead content and phthalates. For the EU, EN71 Part 1, 2, and 3 certifications are mandatory to ensure flammability resistance and the absence of toxic chemicals. Ensure the fabric is breathable polyester or cotton lining to prevent skin irritation, and verify that small parts like sequins or beads are securely attached to prevent choking hazards.