OP, I think you've done the right thing regarding expressing your feelings, which I believe are valid.
I have a very 'keep it professional' view when it comes to workplaces, skewed by past experiences.
There is really - in my opinion - no need for them to be exchanging messages or building any sort of a friendship outside of the workplace. I've been flamed on here before for my views on this topic, but I think investing in these kind of borderline flirtations is playing with fire. This will sound a little dramatic to some, but this is how feelings can start to change and grow.
This young lady probably enjoys hanging with the boys and being doted on, but she's not the married man with a family. It will be entirely up to your DH to set the tone for their interactions.