Get them one by one and get down with them and have them pick up the blueberries one by one (if that is what it takes). Ignore the ensuing 'paddy'. Then 'time for bed'. You show no emotion.
Go into another room and pour yourself a drink and drink it and then you do to bed and say 'calm blue seas, calm blue seas' and visualise a 'calm blue sea'.
I know this will be seen as hardline but this is the way I'd handle it. Don't be mean about it just get the job done. I"m hard but well... thats life, good luck, best, THK