The ideal length of time for a 10-year-old to meditate will vary depending on the individual child's ability to focus and their level of experience with meditation. Generally speaking, it's best to start with shorter meditation sessions and gradually increase the duration over time.
For beginners, a good starting point might be 5-10 minutes per day. As the child becomes more comfortable with the practice, the duration of the sessions can be gradually increased by 1-2 minutes each day or each week.
It's important to keep in mind that meditation should be a relaxing and enjoyable experience for children, so it's best to avoid forcing them to meditate for longer periods of time than they are comfortable with. If your child shows signs of restlessness or discomfort during a meditation session, it may be a sign that the session has gone on too long.
Ultimately, the most important thing is to encourage your child to develop a regular meditation practice that they enjoy and that supports their physical, mental, and emotional well-being.