I think the general sentiment of this post is more solid than anything I have heard. I think it is extremely important for the kids to have fun, if they don't, they won't be around long. But there seems to be some differing opinions on this, so I thought I'd dissect it a little.
Let's separate this out here. We have several areas of baseball/being a kid we are talking about.
You have the situation in which a team goes on an out of town trip. As I stated previously, for MANY families, this is their vacation. I think the kids should be allowed to have as much fun as they want off the field and in their free time.
You have practices. While drills are being done and practice is going on, the kids should be focused. While they are picking up balls between hitters, I don't see it as a big deal that they throw them back into the bucket. As long as it doesn't get completely out of control. I understand that most teams have limited practice time. If they want to goof around a little while they are on their water break, I think that's fine as well.
You have games. While the kids are on the field, they should be focused and ready to play. When they are in the dugout, they should be able to goof around a little bit, as long as they are focused when they get in the box or between the lines.
Activities while not playing baseball. Let them be kids and do all the stuff normal kids do. More than likely, MUCH more of their free time is taken up than a normal kid's would be. They spend most of their weekends at the ball field. They may spend 2 or 3 nights a week at the ball field as well, at practice or pick up games. They probably play in a tournament during spring break and other holidays. If they start feeling cheated about being able to do normal kid stuff because they have baseball the next day or later that day, they will very likely become resentful. One of the things my son still says to this day is he never had a spring break his whole life. 4 years of HS ball and 3 years of college - every spring break has been filled with baseball. I don't think he would have had it any other way, but he knows it's there. We tried to make sure that his free time was fun.
Now I will say this. For some kids, it is harder to turn on and off the focus. Some kids can goof around in the dugout, flip a switch and be 100% focused on the field. Some have a harder time doing this. I don't think the way to deal with this is to cut out all horseplay. There needs to be a balance. Especially for the younger kids, they need to be kids. They need to have fun. Their life does not need to be consumed by baseball. If they start feeling like not only are they taking up all their weekends on the baseball field, but when they are not at games or tournaments, baseball restricts their lives as well, it could become a problem. They will get tired of that.
I don't think anyone is saying that kids should goof around on the field during games, or that practices are just a chaotic jumble of goofy kids running around. I think the point is, don't let this kids game become so serious that it ceases to be fun to do it. Do not let it consume their lives. At the appropriate times, let the walls down. Teach them how to focus between the lines, but have fun outside the lines. Let them be a kid when they are not playing.