Are Belgiums eligible for Chinese VOA in Hainan for 30 days?

Yes, they are. Anyway, you should know that Chinese authorities try to diminish the importance of Hainan VOA in favor of Hainan 30-Day Visa-Free Access. Also, 30-Day Visa-Free Access is better than VOA because in this situation you are supported by agency through which you should book just a simple airport transfer or a half day tour. Simply speaking, there are no issues if you travel through 30-Day Visa-Free Access, but certain a bit unpleasant situations have happened by relying on VOA. If I were you I would be relying on 30-Day Visa-Free Access.