First, I think it is wonderful that you are so interested and involved in your grandson's education.
This is a difficult question because I sympathize entirely with your opinion that children today are, in general, over-scheduled and need more time to "just be kids."
Ultimately, however, you are unlikely to change anything by opposing your daughter's wishes. In any parenting discussion, consider how you can best maintain a healthy relationship with your daughter and your grandson. Your role now is a supporting one.
I do not know the particulars of your grandson's life and his schedule. So, without "taking sides," here is some information and suggestions you may find useful:
Try to approach the idea of play, downtime, and physical activity in the framework your daughter has established. That is to say, emphasize the cognitive benefits of play. After all, the Ancient Greeks and Romans knew the importance of a sound body to a sound mind--and they were no slouches in the intellectual department!
Play helps children process what they have learned, improves verbal communication, and extends concentration and focus. Play also encourages empathy and other social skills. As children play, they learn without even trying.
Your daughter may be concerned that unstructured time will be "wasted." And with the temptation of video games and constant children's television programing, that is a valid concern. Active play, however, is never time wasted in my opinion.
She may be open to the ideas in the books "Unplugged Play" or "The Dangerous Book for Boys," both excellent resources for active play that stretches both body and mind.
Be prepared, however, that she may not be interested in discussing the topic further. In which case, you must respect her wishes as the mother.
At this point, your role is to encourage your grandson to express his own opinions in a constructive and healthy way. Without challenging his mother, he can still communicate his need for some time to relax. He can develop his own plan for how he can productively use his after school time.
If you live nearby, you should also volunteer to spend time with your grandson after school at least once a week. And when he is with you, you can encourage the type of play you believe young children need.
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Candace Lindemann, Ed.M. - JustAsk Expert
Educational Consultant and Writer
http://CandaceLindemann.com