Okay, so my little cousin who is like 7 or 8 entered a science/Inventor’s fair… she doesn’t wanna do a science project so she’s entering the invention side of it. She told me she wants to make a robot dog that plays fetch. I told her I can help her out with it, but when I said this I had the Lego NXT/Mindstorm sets in mind for building it… I thought they’d be like $50 or so I looked them up online and they’re 300$… I need something cheaper and easy to build. Any suggestions?