I went thru FRA last yr to/from TLV, UA to/from US , LH to/from TLV
I got lost a few times as you arrive in 1 area and the next flight is from another, and I live on planes. Besides 11 hrs is a long time to have to kill and if it was me Id go nuts simply hanging in the Term especially if I didnt have Lounge access and I dont think theyd let a 18 & 15 yr old into the Lounges w/o an adult with them
Id try for something with alot less layover time or if somehow they can fly to XYZ from FRA and then to TLV, or aflight that gets in alot closer to the time the TLV will depart
Unfortunately theres really nothing to do or see in FRA, most other Cities theres enough things to keep them busy