504s are a pain for teachers and staff so they'll push you to not file one, but when a hospitilzation lasts longer than expected or it comes time for her to start carrying her own enzymes at school, treatments at school, etc. you'll be glad you have everything official.
Through highschool the biggest thing I actually benefitted from the 504 in school was unlimited no questions asked bathroom breaks. It isn't something I did, because as a guy and in highschool, when you "blow up" the bathroom, it's something funny and often rated on a 1 to 10 scale if someone comes in, but lots of people work into the plan arrangements for you to use a private bathroom. Sound effects and a smell that could peel wallpaper is more embarrasing for most people than me.
Include something allowing her to have a snack and/or drink in class, that's really helpful getting enough calories in. My highschool had a rule about only water in a clear plastic bottle is allowed in the classroom because some students decided that a strong drink makes school go by faster, so for me teachers just kinda looked the other way when I had a soda or gatorade out so as to not make a big deal about anything, it might not work that nicely in elementary school though. I always tried to be descrete, and as far as everyone else knew, the teacher just never caught me with it. Same with food, I would keep something in my bag to snack on all day. (my school decided that lockers were a bad idea so we all carried around our backpacks all day to classes, like somehow thats a better alternative...)
Have some arrangements for missed school and maybe extended due dates while she's sick, people don't realize that when a cfer is sick, we'll still be at school, but when we get home we crash, so homework falls to the wayside. This would probably require a note to the teacher from you, it gets tougher in highschool when most students can sign their parents name better than thier parents can.
search 504 to find what others have done, I know this topic comes up around school time every year