How can I block an outgoing call to a specific number?
On Xiaomi phones running MIUI (their Android port) you can restrict calling to a list of fixed dialing numbers or to numbers with certain prefixes. Similar options may be available in other Android phones as well, be sure to check before you buy a phone.

In MIUI 9: 


open the phone app
select menu ('hamburger' icon on bottom left)
select settings
scroll down and select 'Advanced settings'
select 'Fixed Dialing Numbers'


Otherwise, Android apps for parental control should provide that functionality.