Masterpiece illustration, low shot, closeup of one child sitting on sand with a smile; happy, cute face with extremely detailed big eyes; blonde hair in pig tails; engaging in building a sandcastle with a colorful plastic bucket; dramatic sunlight creating a lens flare; beach scene adorned with scattered shells and hints of sea life; presented in 16k UHD with HDR quality; ensuring a pristine, unblemished visual devoid of technical flaws, focusing on the child's joyful expression and playful activity against the stunning coastal backdrop.