Hi caz having read a lot of your posts and seen your wonderful video I think you are a born entertainer BUT
whilst you are 18 years of age and, realistically, now is the time to be pursuing your dreams before settling down, you should really consider all your options carefully.
As people have mentioned, a lot of the lollipop girls seem to be friends/family of his and from what I can gather, often change frequently. If you go and sign up to an agency then there is a possibility that they will make you do other gigs aas well as Mika's, thats even if you're lucky enough to get a part as a lollipop girl.
As far as I am aware there is no such 'lollipop girl' agency and any agency girls used are from different ones across the country. These girls work some really ****ty hours and like your family have already expressed, work for some ****ty people too so please bear that in mind.
If you are lucky enough to fulfil your dream then I wish you all the luck in the world, you certainly would be fantastic at it!