My baby is only 8 months so the sightseeing will be more for us. Somewhere with a bit of scenery and a bit of shopping also. Would like accommodation to be relatively basic just somewhere to sleep. Baby is 1/2 bf and ff.
But they're in the opposite direction.....
Bergues and Cassel are in the right direction. You can do a walk around the city walls in Bergues and Cassel is on top of one the few hills in Flanders so if the weather is good you can get a good view. But they're not much good for shopping.
You'd probably have to do that in Calais before heading north.