Update a MobX nested observable.map property - mobx

Question

There is a simplified store with an observable.map:
class ThStore {
constructor() {
this.store = mobx.observable.map({});
}
}
mobx.decorate(ThStore, {
_store: mobx.observable,
}
The store holds data with this format (abbreviated), it is a javascript object literal that is managed by mobx as an observable.map. The data itself is a collection of items keyed with its id that has some nested child collection with the same format:
{
"some-id": {
id:"some-id",
label:"some-value",
childs: {
"child-id": {...some properties},
"other-child-id": {...some properties},
}
},
"other-id": {
id:"other-id",
label:"other-value",
childs: {
...
}
}
}
A change in a whole entry in the store works as expected
storeInstance.store.set("some-id", newItem);
But I cannot find a way to update only a property of an item. Getting an item gives a Proxy object which I don't know how to use to update some of its properties.
const item = storeInstance.store.get("childs");
// item is a Proxy instance and has no `.get` method to retrive a child
Trying to update the item like this works, but forces to update all the item and not just the required property triggering more re-renders than necesary.
const copiedItem = mobx.toJS(storeInstance.store.get("some-id"));
// This change does not trigger mobx
copiedItem.child["child-id"].property = someValue;
// This change does but forces updating all the item
storeInstance.set("some-id", copiedItem);
So I don't know how to do fine grainned updates to a mobX store using observable.map with nested child maps.
EDIT: the problem is that the nested childs property is not converted to an observable.map by mobx but is left as a plain object. How can be the nested property be forced to be converted to observable.map?