I am traveling to Rome with boys ages 10 and 13 in March 2016 who have requested a side trip to Pompeii. Most of the tour companies cannot guarantee a climb of Mt. Vesuvius in March. Now I certainly would not go if it were inclement weather, but I understand that Vesuvius is "open" at all times and I think the boys would enjoy this more than the archeological museum which is the alternative on the bus tours from Rome. I am willing to travel by train versus a tour and perhaps hire a guide once in Naples. Are there tours or tour guides that guarantee a Vesuvius climb if at all possible? Thanks.
Some guidebooks list opening/closing times for Vesuvius, I am not sure where you got information that it is "open" at all times. March can be pretty iffy for decent weather conditions, although you could probably get part way up by car. Closing hour for March is 4pm. There are good reasons hikes are not guaranteed at that time of year, never mind that it's been a while since the last eruption. National Geographic has a good video on Vesuvius.
It is open at all times. The only time the path is closed is if there is severe fog. If your day is sunny, then by all means take a side trip to Vesuvius. You can take mini buses from Ercolano station. It costs 20 euro per person, including park admission (Vesuvius is a National Park). Just dress warmly and if the day is cloudy or if it is raining, then you won't see anything at the crater, so the archaeological museum it is.
If your kids will find the museum boring, you could always take them to Naples' waterfront and hire bikes for an hour or so. It's a pedestrian zone and there is a great little castle there that is free to explore.