You must have permission from your parents and enough money. Cupcake shop need a lot of money, because of place to rent for shop, things needed for shop, things needed to bake cupcake and as I learned in business first 3 years for business is very hard and you may not see profit, because there is very big competition and as new shop won't be popular and is small, it will be very hard for it with so many competitors. So your business needs to survive. Pet clothing line also need much money, first of all to make those clothes and then to make an advertisement if you want to just give your cloths (your designed pet clothing line) you need to have high quality and it needs much money and better if you will ask/hire an adult to introduce your clothing line to shops because I think they are more likely to listen to adult and to take it serious. For both of them you need money for materials and also money to pay for workers (who you will really need). So as I said you need permission from parents and much money for it.
To get money you can ask your parents for it but better save your own money, sell things which you don't use at all and don't want to have, or just work anywhere where you can by age and by your own willingness.
If you will sell Pet Cloth on Ebay or Amazon, I don't think that you need any permission of parents and would be way easier for you.
P.S. I am also 15 and good luck with it