We've only done Canvas Hols a bit further up the coast (Fouesnant near Concarneau) but did it with a 3 yr old & a 4 month old & survived! We went back the next year.
Don't know La Baie but Carnac is a sleepy little town with small commercial centre. There is a large Leclerc in Hennebont from memory which is handy. You can get anything in a large French supermarket, no probs. Bestest tip for cleaning is to get some of those (non-environmentally friendly) wipes for cleaning surfaces as with small kids it's a bit of a doddle to wipe over the table & chairs after mealtimes & go out again.
Cots are normal ones, maybe slightly on the cheaper side so if you like a softer mattress for your younger one, perhaps worth taking a small cover/duvet to go underneath. I think Eurocamp provide blankets etc anyway so my two are always very toasty in the tents.
I did take a long bungee cord (embarrassed emoticon) & reins for my toddler at that age & she was attached to the tent on a long leash. Not proud about that but she was at an age when she made regular attempts at escape & that kept everyone slightly calmer...
The ferry from Portsmouth to St Malo is brilliant. We always head for the cafeteria option almost as soon as we get on board (dump stuff in cabin first). Then you sit & eat while watching England slip away from you. Not fancy environment but good food, kids happy & as you don't sail until 20.30, they up late so once filled with nosh, you can give them 10 mins in the children's play room & mini cinema ( Jungle Book & Ratatouille last year) before all heading to bed. We've always slept very well, the beds & duvets are lovely. Have you booked a cot on board for the 1 year old? If not, they can be requested onboard at the info desk.
Back to the cafeteria for breakfast & watch all the boats as you come into St Malo.
The way back is more fun, although you do spend most of the day on board (1030 departure from France). The children's entertainment room/cinema/parade keeps ours happy & we booked a day cabin last time (which was vvvvv good idea). We all had a snooze. I took my 4 yr old to watch Indiana Jones in one of the 2 onboard cinemas, there was face painting etc etc.
Normally the campsites have reasonable shops so you can get the absolute basics for the morning without having to rush off. At an augmented price though.
Hope you have a great time.