The thought goes a long way, but what you come up and are comfortable with doing ... I suggest that you do so without having to be asked ... small things will mean a lot ...

The week she has chemo .. on her first day or two home, you might take the kids out for some activity away ... she isn't likely to feel up to doing much in those first couple of days and the kids may need a chance to get away and release some of their energy and fears ... and it will give her a time when she doesn't have to think about putting on a brave face for them every moment.
She probably won't be allowed to see many people in those days, so you might arrange it ahead of time, it will give them all something to look forward to ... even if it's just a trip to the local park for an hour or so ... or maybe a trip for an ice cream ... something not too out of the ordinary, but a change ...

If she has to stay in the hospital over night, you might show up with a casserole or just a dessert on one of those nights.

A card that arrives during the week after chemo ... something bright and cheerful

If it's the right weather for this, you might get a few flowers that can be planted outside her window ...

that's a start of some ideas anyway ... any thoughtful gesture at such a time will mean a lot!