- Edited
I definitely still think she could ask for the key and do the trials for herself as I suggested above. However, looking back, I think that with the help of good advice on here, she's already established that they cannot force her to wear it, (what would friends/family say? etc), so the risk is negligible, the only concern here is avoiding further bad feeling. Ironically, going through with the original agreement might help to address just that.
First, she wants to find out what it is like to wear it for longer. It may well be that she has had enough after the first time and wants to leave it at that.
As I understand it, the deal was that belt could be removed for any reason during the trial period.
Overall, it kinda depends on her motivation. If she's seriously considering chastity then Ines is right, it's not a fair test of if you have the key in your possession. Further, if the belt is just being considered for use as a toy then it might be kinda unfair on the parents to demand the key.
If however, she is simply attempting to keep to the previous agreement, or remains generally unsure about the whole idea, then the parents could help this by supplying the key for the trial period.