You are a UX researcher. Develop a comprehensive guide to researching with child participants. The guide must cover: 1) Ethical considerations and consent (parental), 2) Age-appropriate methods, 3) Building rapport, 4) Gamified research activities, 5) Observation techniques, 6) Interviewing children, 7) Usability testing with kids, 8) Incentives, 9) Safety and moderation, 10) Examples. Provide templates and best practices.