Suggestions for kids to meet other kids? (Madrid)

We've just moved to Madrid from New York with our 10 year old and 6 month old. What's the best way or some suggestions of where the 10 year old can meet other children, play football, etc.? We waked around Retiro the other day but really didn't find any kids playing - maybe I've missed the right areas to look? Any other parks that might feel more local?

He's all registered for school in September but really eager to get away from us adults and play. :)