I am not old enough to get a job either. What I do is buy and sell stuff on Ebay. It is very effective if you find good deals and work out good profit margins. In the past 2 months I have made £260 profit, which is around $390... It takes some time to get money, but there is barely any work involved... I started off with £80, and slowly build that up and just kept buying more and more items to sell. I hope to keep building my money up and start buying and selling more expensive things with more profit.
It will be a long process, but it is a very good way to make money. You just need someone to set up a PayPal account up, and you're set to go.
Ps. If you are not old enough to get a job, then you must not be very old...? That is very generous of you to think about taking your family on a cruise. If you reach your goal I am sure they will be speechless that you are doing that for them! I would be... You are a kind person :)
Want to add also, I sell stuff on a website called Gumtree here in the UK. I think your equivalent is Craigslist? Stuff sells for much more on a website like that compared to Ebay, so you could sell some on that too, like mostly bigger things? Things that might cost too much to mail on Ebay, so you could just say Pickup only. If you sell phones, never sell them on ebay, you get next to nothing, sell em on Craigslist, you will be surprised what people pay for second hand phones on those sites....