Hannah is right given that your table is so simple, either or as Parent / Subs. It will make your life much easier.
However, if you have as we do, 5 levels of subtypes; that subs of subs of subs of subs, ESRI couldn't give us a way to make it work.
Our "IF Then" section of VBS file has, over the past 10 years, grown and shrank, and been modified again and again.
What I'd like to know is for the next generation of this project would it be better to switch to ArcMobile and use a tablet? Will the VBScript be translatable. Or has there been any developments in the Geodatabase that would allow multiple layers of subtypes?
I'll attach a copy of our lookup table for the forms.