Congrats on your upcoming anniversary! Coming to celebrate that and your birthdays here in Florence will make both magical, for you both and your children.
I second the recommendation about St. Mark's but I'm not so sure about Ognissanti. I would suggest getting in touch with St. James, and both of them can do a blessing in English.
As for a nice restaurant, how nice are you thinking of? You don't mention a time of year, so if it is in the warmer months I would suggest something high up with a view on a terrace, such as Sesto at the top of the Westin Excelsior.
Take a look at some other options here:
Looking for a restaurant recommendation for a special evening meal out? Read the recommendations on special places in Florence to enjoy an unforgettable meal for an anniversary or on honeymoon.
www.visitflorence.com
Head to one of our suggested rooftop terraces or locations for a magical evening enjoying the view from above of Florence's main monuments.
www.visitflorence.com
You also don't mention the ages of your kids, I have an 11, 6 and 4 year old and I wouldn't take even the oldest to the Enoteca Pinchiorri which is like the top of dining in all of Florence. It would just be wasted even on her, because mine are happy with pizza or pasta any day so going to a gourmet restaurant would not be appreciated at all.
My kids would be more for the trattoria, we have a list here of some of our favorites:
DiscoverTuscany list of the the top Trattoria restaurants in Florence, Italy for the best prices, food selection and home styled atmosphere.
www.visitflorence.com
Buca Mario and Trattoria La Casalinga are good options for both Florentine steak for the adults as well as pasta dishes for the kids and still very nice in general for a special meal/occasion. You might try several of them during your stay! I also like Osteria di Giovanni on Via del Moro, for the same reasons. Let me know if I can help with any more questions as you plan your stay!
