like richiavo, I'd be curious about the time of year.
I guess you're aware that Étretat is rather tiny, after 4 nights you'll know it really well. I assume you're further aware that the beaches around are pebbles only (afaik) - the surf makes really nice sounds on them, yet personally I prefer sandy ground for swimming. this said, the coastline is pretty unique, this will make some lasting impressions, in case you like art, your daughter might enjoy compare the works of Monet from Gare d'Orsay to the 'real thing'. Le Havre has an interesting history, too, and the post-war architecture has earned world heritage status by now. busses in the area: http://www.seinemaritime.fr/nos-actions/transport/se-deplacer-en-seine-maritime/nos-lignes.html
if the emphasis is meant to be on swimming/relaxing I'd prefer St Malo, which has plenty of decent beaches. the (rebuildt) old-town and the fortifications are pleasant for strolling around. your daughter might be too old already, however at low tide there are plenty of rocks with pools of water to study sea life (sandals/bathing shoes essential). nice thing to do is a hike to the fort on the small Grand Bé island - yet only at low-tide... as a Canadian, you might enjoy the fact that it's the birthplace of Jacques Cartier. public transport options for going to Mont St Michel:
http://www.saint-malo-tourisme.co.uk/files/ot-stmalo/files/fichiers/pdf/2016/lignes_publiques_saint-malo_le_mont_saint-michel_4_janvier_au_29_mai_2016.pdf
p.s. another thought: Trouville has a nice beach and a good location for excursions (Caen, Bayeux, Honfleur etc.), and it's easy to reach, however it's not as 'unique' as the others.