You can have a leisurely meal in most Parisian restaurants. Without having a better idea of where you'll be, your budget, and how picky the kids are, it's hard to make good suggestions. Creperies usually have something to please everyone. The Crêperie Josselin on rue du Montparnasse in the 14th is good, though the atmosphere there is busy and loud rather than relaxing--same thing for Ty Breiz on boulevard de Vaugirard, but there are others scattered throughout Paris as well.
Here's some additional information that might be helpful. The kids are old enough that they are (generally) well behaved and don't mind a good sit down meal. That being said the finer points of cuisine will be lost on them so it would not be worth it to go "over the top" on a fantastic meal. We will be staying near the Cambronne metro stop.
I'm hoping to find some place that would not be awkward to walk in with kids but isn't a kindergarten either. We are flexible on types of food and budget is "moderate" (I know it's Paris so even moderate is expensive to us, but that's OK for the trip).
One other thing, I know that generally 8pm is early for most Parisians for dinner. We don't need to eat at 5 but anyplace that serves at 7-7:30 would be a bonus as well.
Thanks a bunch!!