Ours don't let noise through either.
You can lock them with keys.
If you left the bottom open a little bit with the restrictors 'off', then yes a young child could maybe push it up further if they were very determined to squeeze through. We don't leave the windows open like this.
If we want to leave our upstairs windows open we normally leave them such that the top half/third is open and the bottom half is completely closed.
Downstairs we wouldn't do this for security reasons, so we have the restrictors permanently 'on' so you would only ever open the windows leaving a small gap at the bottom.
Both ways are safe for small kids and we don't think of them as a safety issue.
In the first (upstairs) configuration, there is no way a child under 5 could push the bottom up from completely closed, unless they are unusually small (therefore light) windows, and the child is unusually strong, in which case you probably want to avoid this and keep to the second (downstairs) configuration.
Regarding our downstairs configuration, the restrictors are about 3 inches above the bottom of the top sash. In our case this is about 1.5m off the ground and safe from kids. The restrictors really need two hands to push and click in to undo them (on two sides of the window).
Hope this makes sense!!