[Bug] with_child_unrestricted not properly removing restriction

### Describe the bug

https://github.com/temporalio/sdk-python/blob/ddbb92fdb052f6e1eeecc25bec37e654eabed4fd/temporalio/worker/workflow_sandbox/_restrictions.py#L311

Should be:

```python
if child_path[0] in self.children: 
```

