It depends on how it is implemented
A VPN is typically used to change the apparent location, if you change to a country that has restrictions then you inherit those restrictions and also change the algorithms underlying data about you which can mean seeing content relating to that country (potentially in another language)
A VPN would grant an IP address within its remit, which can be another source of obstruction such as whether you have an IP address within a block managed by a mobile / internet provider or within a block managed by a VPN provider
Legislation will be directed to the social media providers, they will need to take reasonable steps, which of course could be bypassed with some effort. A child bypassing could let the provider off the hook or could be using a method that the provider should be taking into account
Steps were brought in for access to porn. Children will still be bypassing and accessing it, but internet providers and source sites should be taking reasonable action.
Parents also bear responsibility even if not legal liability
ID on apple accounts is one example, it’s not a legal requirement in every country, but a basic check has been put into place by Apple.
You may not be asked to verify your age with Apple if you have had an account for long enough and have the choice whether or not to provide proof of age - that may or may not impact on your use of services