In websites, it's written a healthy adult missing prayer intentionally is a grave sin. What about young children who have not reached the age of adolescence (i.e. puberty)? Is there sin on them? Please provide Hadith or Quranic verses to support this answer.
A child is not subject to the obligation of prayer, and will not get any sin for missing it. They are only told to pray for purpose of education and to instill the habit.
رفع القلم عن ثلاثة عن النائم حتى يستيقظ وعن الصبي حتى يحتلم وعن المجنون حتى يعقل
The pen has been lifted from three: a sleeper till he awakes, a boy till he reaches puberty, and a lunatic till he comes to reason.