Children make up a very small proportion of reported COVID-19 cases, with about 1% of all cases reported being under 10 years, and 4% aged 10-19 years. Children appear as likely to be infected as adults, but they have a much lower risk than adults of developing symptoms or severe disease. There is still some uncertainty about the extent to which asymptomatic or mildly symptomatic children transmit disease.
Update: Recently (April 27th) some rare syndrome has been seen in UK children, and the WHO is investigating whether coronavirus causes rare inflammatory disease in some kids. Stay tuned as we will update when we know more.
Source: ECDC (European Centre for Disease Prevention and Control)