Well obviously you are not going to get suggestions for malware - that would be a clear violation of the forum rules (and if there were any Android "viruses", i.e. self-propagating malware, that would be an exceptionally irresponsible thing to distribute as that would risk infecting other devices. Luckily while there is Android malware this is not of the type that is properly called a "virus").
A child is presumably not living on their own, so can the person or people looking after them do nothing? Bear in mind that if they disagree with you, feel there is a need for the child to have a phone, or if this action causes problems for them, they may simply provide the child with another phone which you have no access to, so this action may well be futile and possibly damaging to your relations with the child or their carers. There are parental control apps that can limit usage, and maybe discussing those with the person or people currently caring for the child will be a more effective approach.
