The parent is the source of truth, so the child needs to tell the parent to change its state. Pass a callback from the parent to the child, and have the child call it.
There's an example on communicating between components in React's docs that might be helpful too.
A = React.createClass
getInitialState: ->
mystate: {test: 1}
incrementTest: ->
@setState {mystate: {test: @state.mystate.test + 1}}
render: ->
myChild {onChange: @incrementTest, param: @state.mystate}
myChild = React.createClass
render: ->
React.DOM.div {onClick: @change}, @props.param.test
change: ->
@props.onChange
ajax("/..../", JSON.stringify(@props.param))
.done(@onDone).fail(...)
onDone: ->
console.log "Hum..."