Amazon is your friend. You can buy everything in less than 10 minutes (*except for latest gen consoles, in which case you are going to have to go to Kijiji or eBay).
13-year old should get a PS4 or XBox One (PS4 with a Vita and a PSN subscription if I had to choose).
The girls should get 3DS's or 2DS's. You may want to consider a WiiU for games like Skylanders or Disney's Infinity, although I am not sure how much they would like / hate the gamepad.
The 7-year old would be tough. I think most of the games on a PS4 / XBox One would be too hard for him, but I can see him wanting it if he sees his brother / cousin playing it. Otherwise I would recommend a 2DS (for Pokemon) or a WiiU for Mario / Sonic games.
FYI, this is based on my experience with my own daughter and nephew.







