Bay of Fundy/Moncton/St. John, NB Itinerary Advice
 

We will be arriving in Moncton from Montreal by train Monday July 6th (about midday) and to return to Montreal on Sunday July 12 (late afternoon). Two adults and two tweens on the journey. Our plan so far is to visit Shediac and do the lobster boat cruise the day we arrive in Moncton. Next day (on the 7th) we arrive in St. Andrews for 3 nights and plan to leave on Friday July 10th.

We'd also like to see St. John, Hopewell Rocks, Fundy National Park, maybe the ferry to Digby. These other things seem like a stretch with three nights in St. Andrews. Also, no time really budgeted for Moncton.

Questions:
What are the must sees for our limited time?
How much time should we budget for Hopewell Rocks/Fundy National Park? Is it better to stay in Alma for a night?
Should we cut our time shorter in St. Andrews for the other spots? It seems like such a pretty place and it's nice not to be constantly unpacking. OTOH, I don't want to spend too much time driving back and forth to destinations.
We haven't budgeted much time for Moncton or Fredericton - should we?

Thanks in advance.

Of the destinations you refer to, I've only been to Moncton, so the only question I can answer is "should you budget time there". I like you took the train there from Montreal, but my friend and I did not rent a car, so the situation required that we spend the night in Moncton on our way back from Charlottetown (after a cycling trip to PEI).

I enjoyed hearing the Chiac French (quite different from the Montreal variety), going to the Saturday Market, seeing the Tidal Bore and found a few restaurants I quite enjoyed. I thought Main Street nightlife had a surprisingly vibrant energy (sizeably francophone, too) for a town its size. That said, you'll see more dramatic tides at Hopewell Rocks, you'll find good restaurants in the other destinations and you'll hear Chiac in Shediac. Consequently, I think it's ok if you choose to skip Moncton to head to the natural attractions, but I did find my morning and afternoon interesting there.

Allow for the possibility of the train being late. Also time to process paperwork for the car.

I would go to Alma first, an hour drive from Moncton. You would be on your own timetable. Spend a night in Alma.
Spend as much of the next day as you wish in Fundy National Park, then head to St. Andrews (2.5 hour drive) for 3 nights. You can take a day to go to Campobello island, etc. As you say, pretty area.
Leave St. Andrews for Fredericton and see the Beaverbrook Art Gallery. You could if you wish push on to Moncton for the night (3.5 hours from St. Andrews), making it two nights there. Either way, head to Moncton for your last night. Take your lobster boat cruise either that afternoon or the next day before taking your late afternoon train home.

You can tweak this itinerary to suit (eg. 2 nights in Alma, drop a night in St. Andrews), but I would drop the Digby ferry.

We offer a couple of tips for New Brunswick, based upon our trip in September of 2014. We highly recommend seeing the tides at Hopewell Rocks. It will be very convenient if you stay at the nearby Hopewell Rocks Motel & Country Inn, which is very comfortable (book a room in the “adult building”; it is quiet and has a Bay view). One day is sufficient for Hopewell Rocks if the timing of your visit coincides with both high and low tide in the same day. A must see in the area is Cape Enrage. It is a set on high rock bluffs over the Bay and has a zip line, rappelling wall, and breathtaking views! Be sure to see the “Tidal Bore” in Moncton (which is an easy drive from Hopewell Rocks).

We chose not to go across the Bay to Digby simply because it was going to be too time consuming.

Enjoy! The area is lovely...and very, very quiet!