To set the value of the flip_h-property you would do:
get_node("Sprite1").flip_h = true
To get the name of a property you can hover with the mouse over its entry in the Inspector. The tooltip will display the name of the property you can use in your code.
Alternatively you can get the i-th child of a node like this:
get_child(i)
Note that i is zero-indexed, so i = 0 will get you the first child node.