I am not quite sure that I correctly understand the case.
Basically, if you show a modal form, you will not be able to select other forms in your application unless you close the modal form. If you show a form in a normal mode, you will be able to select any other forms in your application, as well as all other controls in them.
In your scenario, I presume you could simply disable all controls on your parent form except the text box and enable them when you close the second form.
I hope this is helpful.
the Telerik team