Unfortunately you cannot purchase access to any lounge in T2. Do your parents need visas to enter Schengen? If so their options are somewhat limited as they will have to stay airside. However there are many cafes and restaurants as well as Napcab sleeping cabins in the T2 secure area.

If entering Schengen isn't a problem there are many more options in the area between T1 and T2.

You may also want to check the following threads in the Germany forum.
http://www.flyertalk.com/forum/germa...ounge-use.html
http://www.flyertalk.com/forum/germa...s-transit.html