I would be less worried about how much she is eating and more concerned about her drinking (fluid intake). Does she have a fav drink you can offer her? Even if it is watered down fruit juice, cordial, or flat lemonaide. The sugar in those drinks will help keep her energy up to fight the bug - but be careful not to give too much sugar as that may aggrevate lose bowels.
If she is showing signs of dehydration or has become limb (or you have difficulty waking her from her sleep), you need to go straight to the hosp.

My DD had gastro a few weeks ago and hardly ate anything for a week. She's still making up for it now

To try and limit DS from getting it, strict handwashing techniques are vital but also try go keep DD away from him to limit his chances of getting it.

HTH and hang in there

MG

ETA: The prefered foods for them to be eating at this point are
Bannana
Rice
Toast

There are others but these are the only ones I can recall right now. Banana is the top food for them to be trying to eat if they can.