If you plan to wear a wrap dress to church, and it is likely that you will be juggling a small child on your hip throughout most of your time there, then I highly recommend you double knot the ties. Perhaps you might consider safety pins as well.
If the ties should happen to come undone (failure to heed my advice or simply an extremely active child), I highly recommend that you put the small child down before attempting to re-fasten the ties. If you don't, you will very nearly drop the child and this, although thrilling to many small children, will cause the woman sitting behind you to audibly gasp. And you will be embarrassed, not just because your dress is coming undone, but because there was an obvious witness to the whole comedy.