Is St Sulpice an ok place for kids?
St-Sulpice is a beer garden; loud, hectic, "party" all the time; but not too much over the top.
I'm not certain of the regulations, I don't know if it is a "bar" or something else.
How old are your kids ?
if you want a drink on a terrasse, I might suggest something on St-Denis but north of Sherbrooke; it's nicer and maybe more "familly" oriented.