I have an 11 month old (my own daughter) who has started screaming very shrilly for fun or when she gets frustrated; I have been ignoring it, but now two of my daycare kids (19 months and 21 months) have starting copying her and I can't take the shrieking in stereo (they usually do it at the table during meals)! I have tried saying no to the two older kids, but they continue and of course the 11 month old continues as well. Any advice on how to stop them from screaming???