Paypal Form: How to hide options until checked

Please see attached image.

This is a PayPal form where the user selects number of participants or shirts.  

I would like all the options to appear like the T-Shirt - Short Sleeve (highlighted in image).  The user selects that product then the available sizes  appear.  

I've compared the settings for all the check boxes but cannot figure out what I did when I created it to be that way. :-/

Thank you in advance!

Disregard...I think I fixed it myself (although the fix doesn't make sense).

In the Payment Wizard, the Quantity is before the Size Option, for both the T-Shirt Short Sleeve and Long Sleeve.  

I changed the Long Sleeve to be Size, then Quantity.

That fixed it! (?)

Even though the Short Sleeve shirt was displaying correctly, I re-ordered that as well - for consistency.

Glad that you have fixed it. 

If you have encountered some other problems in the future, please let us know. We'll be happy to assist you.
Regards