In the U.K., children can't just say no.
Kessel
R
R
D000293:Adolescent; D002648:Child; D002651:Child Advocacy / Q000331:legislation & jurisprudence*; D002675:Child, Preschool; D006801:Humans; D007258:Informed Consent / Q000331:legislation & jurisprudence*; D033222:Judicial Role; D033242:Minors; D033282:Parental Consent; D016312:Treatment Refusal; D006113:United Kingdom