You may run into resistance since she hasn't been challenged on a regular basis. I'd explain that there will be challenging math every weekday and emphasize how important attitude is to learning. I put much less emphasis on the results/right answers but strongly praise effort/attitude/perseverance/working through frustration.

Finding something to challenge her should be less of an issue than working through any resistance she's built up towards stretching her brain. You'll probably want to have a few different resources for challenging math in order to find a good match.

Be firm about the requirement for challenging math but flexible with your curriculum! wink