I think you need to define the MasterType so that the child page knows what class the Master Page is. This will make the Master Page strongly typed.
See MSDN documentation.
<%@ MasterType virtualpath="~/Masters/Master1.master" %>
Also, I'm not sure if controls declared on the ASPX page are by default public. So you may need to add them as a property on the Master Page so that they're publicly accessible from other classes.
public SqlDataSource MasterDataSource {get {return this.getButtons;}}