S.H.
Make a LIST... for your Husband... on your childrens routines & shcedules. Daily if need be.
A "Man" will need that... to refer too. They can't remember everything like a Mom. They will forget.
So, type it out... and put up the list in a place he will see it and refer to it.
When I was in the hospital for my c-section... I was not home for 3 days. I made a list of things, for my Husband, on what to do and my daughter's routines and what not. He was home with my eldest daughter, plus with Grandma. He DID refer to the list I made and it came in real handy.... I even listed down for him to water my plants...
So that is my suggestion.
AND, talk about it with your Husband, before you will be away... openly and nicely. Let him ask questions if he is not sure about something.... explain to him what to do with the children... and NOT to just 'expect' Grandma to do everything.
For your child in the hospital... she will probably be in the ICU... yes? Because it is heart surgery. My Dad had heart surgery. SO, there are rules on that and what can or cannot be brought in. Sometimes, visitors are not even allowed in, nor 'children' visitors. So ask the Hospital or Doctor... what the rules are for the ICU unit.
If allowed by the hospital, Maybe bring her lovey if she has one... or any favorite things of hers, so she feels comfy and comforted.
Bring some books you can read to her.
Bring your cell phone.
Your daughter, will be in recovery for those 4 days... and may be tired nor not even want to "play." So you need to gauge her... and ask the Doctor for some ideas.
For your kids at home... you AND your Husband talk with them. Explain simply, what will happen, you will not be home, Daddy will take care of them... and they STILL will have their regular routines etc. Tell them to help Daddy too... and to take care of each other....
When I was in the hospital for my c-section, my daughter was about 3.5-4 years old... and I told her "Help Daddy okay? If he doesn't know something... you help each other and take care of each other... " and she felt real proud... and even told my Husband what time she is supposed to go to bed. Because, even that detail, my Husband "forgot" since "I" was not home. Men.
So... encourage that everyone help take care of each other at home in loving and helpful ways... because Mommy will be at the hospital taking care of sister.....
all the best,
Susan