Assuming your going from Villefranche, Nice is about 15 minutes one way & Monaco (Monte Carlo) is about 15 minutes the other way by train. The station is 5 minutes walk along the front & is about 5 or 7 Euro Return. The trains run approx every 30 minutes each way. This is a shed load of money cheaper than the ships tours if there is a group of you. Good Luck
Monaco is Monte Carlo .
Or you catch the #100 bus in either direction. Would a 9yr old be interested in the corners of the formula 1 race track in MonteCarlo?