Please help!! WHAT SHOULD I DO!?

I live in a house with a big yard,with my parents and my grandparents.we have many animals:2 dogs,one cat and some hens.my grandparents bought some chickens,they are just one month.my grandpa built a little "house" for them,but the problem is that my cat can get in really easy.i'm sure my cat will start to kill them and eat them,because they are little.i told my family about this,they keep saying that our cat won''t kill them.but i saw my cat killing doves and birds bigger than these chickens.and my cat is 9 months old,he's very playful,and he loves to kill smaller animals than him.i hate that!i suggested to my family to keep my cat in the house until our chickens will grow up enough.we are already keeping our cat in the house,but during the day he's staying where he wants,and always outside.they don't want to listen to me.i told them everything,they don't want to build a new and better "house" for those chickens,and i can't ,i'm a girl,i don't know hoW to build,and i'm at school..what should i do?PLEASE HELP! :((