CDG to Nice

It might be enough. You'd want three to be safe.

It will be a lot faster and possibly less expensive to fly. You are already inside the airport. That's a 7 hour train ride with a connection to a local train in Marseille that has no reserved seats. The flight is one hour with a sensational landing out over the water with the mountains sparkling on the other side. It's a wonderful flight especially with a window seat.

That’s what we’re thinking of flying to Nice. EasyJet only has an early-morning flight on that day. Air France is quite pricey for the three of us.

Two hours is a tumble of the dice, my fellow Canuck. We've opted to fly down on AF then take a TGV back up to Paris later in the month. Note that the latter are both direct to Gare de Lyon and surprisingly costly.
Good luck.
I am done. the transit

2 hours is a bit tight. I'd be more comfortable with a 13:15 train or later. I'd bite the bullet and fly.

As for cost, look into whether you can change your original ticket to include the segment from CDG to Nice. It might not be as much as booking a new separate ticket. And if on the same ticket you will be protected for the connection time.

If you do take the train, check your fare conditions for one that can be rescheduled, if necessary. A flight connection can be easier, but just allow two hours because the AC flights and the connecting AF flights can be in different terminals.

Yes, flying is your better option for the same day. It is a BIG airport.