SOOOO cute!!!! I used to have a little slider turtle when I was around 10. But I stopped paying attention to it and my dad gave it away.

Make sure you clean their cage A LOT, and put a little rock in so they can climb out of the water if they want a dry place to sit. I don't know about slider turtles, but I know with some turtles their size depends on the size of the aquarium, so I'd do a little research on that.
Congrats!!