I'm seeing trains for June 6 on both the SNCF and Renfe websites. For some reason I'm seeing all six of you being charged the same price on Renfe, whereas there are modest discounts for one or more of the youngsters on SNCF. I don't know what's going on there. And I'm disappointed the discount isn't greater.
These are the fares I found, first for one adult and then for the entire family (latter from SNCF only):
08:10 - 14:49 with change in Perpignan: €84; €389.70
10:10 - 16:48: €111.70; €536.10
14:00 - 20:48: €89; €427.20
16:10 ; 22:49; €69; €331.20
That first routing can only be purchased from SNCF, but that's fine, because the Renfe website is a lot harder to deal with.
The fares on the last two options look pretty good to me, but those are "Essential" fares, not full fares, and I think they may sell out, leaving you needing to pay more.
I encountered some anomalies on both websites as I checked these fares. The best I can tell, there are a few lower-priced tickets available, but not enough them to allow you to buy six. I saw single-adult fare of €69 on the 14:00 train and €59 on the 16:10 train. If you don't feel you all have to sit together, would could try buying 3 tickets and then another 3 tickets. Obviously, those few lower-priced tickets could be snapped up by others at any time.
I'll update this message after I've had a chance to check out the day-trip to Mont-Saint-Michel.
Edited to add: Don't forget that it's going to cost more to get to and from airports than to and from train stations.
Edited to add -- FOR THE TRIP TO MONT-SAINT-MICHEL:
I found these outbound options on June 11 (fares for all six of you):
7:06 - 10:20: €197.50
7:32 - 11:22: €121.50
7:35 - 11:00: €270.80
9:52 - 12:55: €216.80
10:52 - 13:55: €153.80
14:52 - 17:55: €288.80
And for the return on June 12:
10:05 - 13:09: €216.80
11:20 - 14:52: €216.80
14:05 - 17:09: €274.50
18:05 - 21:07: €157.50
I've been to that part of France three times, twice in the last 3-1/2 years, and I haven't bothered to go to Mont-Saint-Michel yet. Obviously, I can give you lots of reasons not to go:
- It will cost a lot of money
- It's not a place that takes a huge amount of time to see, so you're swapping large chunks of two days that could otherwise be spent in Paris for a very few hours on Mont-Saint-Michel.
- The place is crowded during day-tripping hours, so you'll want to do your sightseeing in the evening and early morning. Without a car, you can't make effective use of the rest of your time on those two days.
- There are many interesting things to see in Normandy and Brittany. You could see them, along with MSM, on a later trip.
- On this forum, several people have commented that they liked the view of MSM from the mainland better than actually seeing the island. In that case, I'm tempted to say, "Why not just look at a picture?"