Give them a ring / or wait for the bill and then dispute.
In my opinion.............
Asking for a quote (presuming you were very clear that it was just a quote) is not a contract to carry out work.
Presuming you did not discuss terms / costs / times / accessing your property etc
Your 15 year old is not old enough to authorise work to be carried out - and he is not the householder.
Alternatively you could check how much it is, and if the work is satisfactory and within the costs you anticipated - Pay them and never use them again.
Sort of an example
My local garage do MOTs for me. If any work needs to be done, they ring with a quote. They will carry out work needed ONLY IF I give authority. I have the choice to go wherever I like and if they dared to carry out unauthorised work, (depending on what was done), I might ask them to undo what they 'fixed' and take my business elsewhere.
Good luck
