Itinerary help: 7-8 days in/around Nice with teenager

I'm not sure there's good public transportation from Nice to Valensole or from Valensole to a convenient rail station. The Avignon TGV station would probably be your best departure point; from there it's just about 3 hours to Paris, whereas the trip from Nice takes nearly 6 hours by train. If you can manage to get to Valensole from Nice, you'd do better to go on to Avignon afterward rather than returning to Nice. However, I'm not sure Valensole is manageable from Nice without a rental car.

I participated in an excellent food tour in Nice in case that’s appealing to both of you. I’ll just go ahead and add the link to that trip report, so you can see other details. I don’t recommend the hotel now where I stayed. It was completely fine when I was there but has since turned into some type of odd arrangement with low reviews.

https://community.ricksteves.com/travel-forum/trip-reports/our-combined-solo-couple-vacation-in-france-2019

Our last trip to Nice was for two weeks and we did many day trips. Nice is the best location for day trips of any place we have visited in Europe. So easy! Most were reached by bus that drove along the waterfront. Easy, frequent, inexpensive. We used the train one time to visit Antibes which we loved. Our other favorite was Cap Ferrat with Villa Ephrussi and St-Jean Cap Ferrat. Least favorite was Monaco but worth a half day if you haven’t been there before. Princess Grace and Prince Rainier’s tombs are in the Cathedral there.
This area including Nice is in the Côte d’Azur.
As far as being safe, we avoid staying near the area around the train station. We love Nice with its cultural mix of Italian and French.
Last time we spent two weeks in an apartment near the classic Hotel Le Negresco. We were a block or so from the water, nice neighborhood. I have never felt unsafe in Nice other than around train station.

We have friends who live in Antibes so we have used that as our base on two trips we have made to the Riviera. We have taken the train in both directions from Antibes in order to see Cannes and Monaco, have taken a ferry to St Tropez, and our friends have driven us into Provence, up to the hill towns around the Riviera, over to Cassis and Aix, over into northwestern Italy, and up into the Maritime Alps above the Riviera.

There are sometimes pickpockets working the local trains along the Riviera, so be careful of your purses. If someone gets into your personal space unnecessarily, assume it's a pickpocketing attempt.

A trip to Cannes might not take the full day. You can combine it with a ferry trip to Ile St. Honorat, an island off the coast with a monastery, winery and some gardens. Or you can combine it with a beach day or a trip to Antibes. Antibes can make a good day trip, and you can make a beach day there or in nearby Juan Les Pins (the public beach near the Golfe Juan side is usually popular with local youth). That is one stop beyond Antibes.

Another potential trip is to St. Paul de Vence or Menton.

If you want to go to the Valensole plateau, you can take the TGV to Avignon and then take a day tour form there. (the TGV is outside the main city) You can stay overnight and then take the TGV home. There is also a FLIX bus available. Or you can do a one way car rental and directly stay in the Luberon or Valensole (drop off in Avignon, at the TGV)