Many questions you ask at once:

Here a map first: http://www.viamichelin.com/viamichelin/gbr/dyn/controller/mapPerformPage?strCP=Provence-Alpes-Côte%20d'Azur&strCountry=000001424

Nice would be right in the middle. So convenient to visit Cannes as well as Monaco-Monte Carlo.

A car would be quite practical. Are they coming this week-end? If yes, careful, there is a NATO meeting in Nice and it is chaos. If it is next week-end, no problem.

Hotels should be no problem as well as it is low season and holiday time, many people are on the slopes. Too cold for the beach, though.

http://www.cote.azur.fr/hotels-ville...ce_lang_en.htm

Car hire: http://www.france-car-hire-rental.com/

In order to save a little money, they may want to get the car in the city instead of at the airport.
Tell them to go to St. Paul de Vence, beautiful village and take the Corniche road to Monaco. Go up to the observatory, absolute fantastic view of Monaco, monte-Carlo from way up. Breathtaking.