If the flight is delayed (either due to force majeure circumstances or due to LH fault), LH will take care of its transit passengers. The options will be:

1) an overnight in FRA at LH expense (hotel+ food. Usually, the hotel is "Intercity Frankfurt Airport"; the dinner voucher is worth 20 EUR plus buffet breakfast and a free transportation to/from the airport) AND a free re-booking onto the next LH flight to your final destination. However, in this case, it is the responsibility of a passenger to take care of all visa formalities when entering Germany.

2) a flight on a partner airline to your destination (either a nonstop or with another transfer). If the flight on a partner airline is not available, you can ask for a re-routing on any airline (however, this option requires good negotiation skills on part of a passenger).

If you are concerned about this transfer, it might be a good idea to prepare a list of possible connections from FRA to your destination the day before traveling and give it your niece so that she could be more prepared.
In any case, 2-hour transfer in FRA is more than enough. LH might, in rare cases, wait for connecting passengers in case many of them are late for a flight in question.