When my children were little, I used to secure buttons on their clothing with dental floss. There is no need to remove the button or existing thread. Just thread a needle with the floss and go through the shank or thread holes of the button once or even twice and tie it off. If it's a dark colored button, you can color the floss with a permanent marker to make it less noticeable.
You can't break dental floss with your bare hands and the buttons will stay on until you want them off. This is especially helpful on coats/jackets and jean waistbands. No more lost buttons!
Source: My Mom
By Carol from TX