Gone through this very sitch.
It's very hard for the parent to know what they need, or to ask for it, especially when it is all new. I had no idea what I needed, what I could and couldn't handle, no idea...
What you're doing so far is great. Most anything that you do will be appreciated -- because the parents will see that you care about them. That in itself means so much.
Here's what people did for us that I found especially helpful. My bestie arranged all of the dinners for treatment days -- she did a sign up, kept a calendar, followed up with folks who'd signed up to make sure they were able to follow through (if they couldn't, she made dinner or found someone else), stopped by the house to pick up serving bowls and take them back to the folks who'd brought them.
Treatment day was also trash day , so our neighbors took our trash to the curb and put the cans back at the end of the day.
One friend checked with me before her weekly shopping trip every week to see if I needed anything, and got it for me.
When my folks came out, one friend picked them up at the airport, and another drove them back.
My husband's friends took him out for guys' nite to watch sports or grab a beer.
One friend sent me a card at the start of every month just to say she was thinking of us.
Lots of people asked after our boy, which I appreciated, but what I really appreciated even more, in a way, were the people who asked how I was doing.
It goes without saying (but so many people don't have enough sense to know it) that offering medical "advice" without being asked for it, or sharing your own personal story about cancer (I did not want to hear anyone else's sad story) is not helpful. If you have been in the same situation, then it's good to offer support and then back off -- "hey, this is a really hard thing to go through, let me know if you want to talk".
We have only one child, but my observation of families at the hospital (and having been one of the well siblings of a chronically ill sister)
leads me to suggest that if there are siblings, that you could offer an outing for the kid(s) who aren't sick, a play date for those kids, offer to pick them up at school, take them to piano lessons, etc. That helps the parents and also can make the well kids feel wanted.
You are so kind to help and support your friend, and to ask what more you can do. You're a good friend, and a good soul!