What is the bear minimum amount of water for a 9kg child. i know books say 30ml, but what is minimum to sustain without becoming severely dehydrated?

Over what time frame: I'm not sure what you are asking here when you say bare minimum. Are you wondering at a time or over a certain amount of time, say a day? I'm not sure where you are seeing 30ml. For a 9kg child, you would want 900ml in a 24 hour period, which is 30 ounces, not 30ml. If concerned about dehydration, you can give tiny sips of 5ml at a time to try and keep child hydrated, about a tsp full.