Is there anyway your parents would allow you to go to Wal-Mart or a pet store and get another 10 gallon tank? They come pretty cheap and a filter shouldn't be too much either.
Have you tried explaining to your parents how important it is that your fish have more room? I adopted an Angelfish a little while ago; he lived in a 2 gallon tank for the first 2-3 years of his life and was healthy. I put him in my 90g and for a good week, he barely swam at all. Then he slowly got used to the open space and is now living very happily. He's much more active now and has developed an amazing personality. I won't lie and say he was dying in the 2 gallon or anything like that, but I will say that his behaviour changed for the better once he was in a larger tank. I think he's very happy to have more space and his actions show me that.