They would fill the feed bins with sand and then top them off with grain and meal.
Both of these requests are similar in the sense that they both involve parents asking others to monitor their children and be attentive to what they are doing. Polonius wants someone to watch over Ophelia, while Claudius and Gertrude want Rosencratz and Guildenstern to watch over Hamlet. Shakespeare wants to show how, even though both of these are requests of parents who want their child watched, the intention and consequences in each case are very different.
