Offer kids choices for when they are going to practice. Create a plan and allow them to have some input. With young kids I find allowing them to pick from no more than three choices is helpful.
"Through choice, you can help students self-differentiate their learning so work is more appropriately challenging. You can also combat student apathy, helping students connect with their strengths and interests and giving them more autonomy, power, and control over their work, which boosts their intrinsic motivation." -ACSD.org
So ask them, "Would you you like to practice when you get up in the morning, right after school, or before dinner?" Maybe they will offer an appropriate alternative (and maybe they will just be silly!) but try to include them in the decision making process to make things easier on you. If necessary you can plan a bonus activity for after practice is complete- like screen time only after piano time. And once you have a plan, stick to it!