An officer cannot just enter property and take things. Even if he witnessed the kid riding around the neighborhood on it, he couldn't take it. Depending on your city ordinances, it may or may not be unlawful, but the most he could do is write a summons. Most likely, the officer would just tell the kid to stop riding it, or talk to the parents about it.
Which is basically something you can do yourself. Stop relying on cops to do your dirty work.