So here are some suggestions:
- If the mothers light the candles after the Processional
- After a reading, or between readings
- Between the vows and the rings (a love song here makes perfect sense)
- During the candle lighting, or whatever symbolic ceremony you have chosen
- During the Sign of Peace
These suggestions are merely a starting point. The one caveat I do want to mention is to refrain from having music while people are speaking. It generally sound like a mistake, and could hamper your guests’ ability to hear what is being said. I know, I know, you see it all the time in the movies, but there are many manipulations that can be done so that seems natural. If you really want it, add it later to your video!