I'm going to cost my family 350 dollars
What will you do to cost your family 350 dollars?
I'm going to take them out and bring them back after the weekend. Bu I am using their credit cards and not mine.
It is good to special trips for your family. It might help to save some money before so it is not a large financial burden just on them and not you.
It's fine, I want to buy some stuff for myself.
You can treat your family as nice as you want but not at their expense. That is not so nice. Add your own money to it and split the trip so it is more fair to all.