For sections of your video that highlight a birth or a newborn infant, there are a number of approaches you can take. For a traditional video try lullaby music. For a funny spin on a birth you can use "Welcome to the Jungle" by Guns and Roses. For a sentimental feeling try "What a Wonderful World" by Louis Armstrong. Other selections might be "Today is the Greatest" by Smashing Pumpkins or a medley made by cutting together bits of songs that use the word "baby." Music from the 1950s is notorious for such use.
As baby grows you will have plenty of photos of the kids having adventures, meeting accomplishments or just interacting with each other or the world. A great song for photos of a female child would be "Every Little Thing She Does is Magic" by the Police or "Beautiful Girls" by Sean Kingston. For sisters "We are Family" by Sledge Sister or "Girls Just Wanna Have Fun" by Cyndi Lauper. For photos of a male child, try "My Guy" by Mary Wells or "My Boy Lollipop" by Millie Small. Other songs great for children are "You'll Be in my Heart" by Phil Collins or "These Are Days" by 10,000 Maniacs.
For photos of mom and dad or anyone who is in love there are so many choices for music. Of course, the best selection would be the song the couple considers to be "their song" or the one they had their first dance together to at their wedding. This effort will take some research. Otherwise, you can try "Wonderwall" by Oasis, "Can You Feel The Love Tonight" by Elton John, or "I Can't Help Falling in Love With You" by Elvis Presley. Other choices are "Crazy in Love" by Beyonce or "Can't Smile Without You" by Barry Manilow