My DD got gastro from the local swimming pool. EXACT same symptoms as what you are describing. I took her to the hostpital and they monitored her. Basically just watched her, didn't really do anything.

They told me 10mls of plain water every 10 minutes. You can also dilute it (half/half) with a bit of apple juice for some sugar if you are comfortable doing that.

But if you have a syringe (like that comes with panadol), it's 10mls every 10 minutes. That's all we did at the hospital.