When DD2 had rota virus it was the 3rd day I took her to the gp. Sent me home again & said to keep an eye on her...5th day she was admitted with NGT for fluids, but they said she was well hydrated considering how sick she'd been.
I think they recommended to me 10 mls every 10 minutes. Bottle, cup, medicine syringe, ice. Anyway you can get it in

We had to go lactose free for a while after that, but rota virus is a pretty strong bug & it strips away the lining of the gut from what I remember.