How to get slimmer when my Mum wont let me diet?

Hi, Im a 14 year old girl, im average size, and I want to diet. Not like dont eat anything kind of diet, I would never do that, just like eat healthy stuff and cut down on fatty foods. BUT. My mother disapproves. I dont know why, but she makes me eat unhealthy food. I suggested to her that I just wanted to slim down a little bit for our holiday (A tropical one, I will be wearing a bikini) but she said no.I want a toned/nicer stomach, and I only have 87 days. How can I do it when my still insists that I eat unhealthy food?? Please help. Thank you :)