'ignorantia legis neminem excusat' Ignorance of the law is not a defence. to 'believe' or 'presume' dosn't always cut it either in the Law Courts. The law is complicated and there is not a 'one size fits all' answer. For example if you are in fear of you life and believe you are in mortal danger you can use reasonable force to protect yourself. There are contradictions in law, but case law sets the standard, a stated case is therefore the benchmark for advocates.
However, In England, Wales and in Northern Ireland a minor is a person under the age of 18. At the age of 18 you become an adult. The law is the law (ipso facto) like it or not.
If you are in doubt of someones age and think they may be under the age of of 18 then it is reasonable to ask them to produce some evidence that they are 18. There are a few cases whereby former model 'mother' has supported daughter. If there is a legitimate reason for this work to be carried out then parents, guardian should be onboard and present.
Yes, it goes on, underage girls do pose but it's a question of ethics and professional / personal judgement. What happens when the protective father finds out that their demure little girl has posed? What happens if she is an attention seeker, alleges assault etc etc?. All it takes is one word to ruin your reputation, which in most case is important to people. Mud sticks and people love a good scandal.
Lots to think about and as adults we have a legal and moral duty to protect minors. I think you need to do a lot more research here and certainly ask a few questions.
Personally, I would avoid but that's just my opinion. Let us know how you get on and what the outcome is?