Perhaps you need to rethink your holiday arrangements. Three weeks is a long time, but you did say 'this year' so it's not set in stone.
Think about what you would like to happen. Could you perhaps use the time to get away on your own for a bit, even if you just drop the children off and go back to your own house. Would you prefer that the children only stay for 1-2 weeks? You might want to take the children away yourself.
Re the regime at grandmothers house, I'd say you have options. Either accept it and let your children know it'll be back to the normal rules at home, or try to negotiate with your MiL (?).
Look at all your options for the future and make decisions based on past experience and a little more of what you want.
July 23 Limerick (continuation of July 21)