Help—back key hole opening

Hi! I recently made four matching garden party dresses for my four daughters out of cotton lawn and they love them and I love them and I can’t wait to make more! But—I’m having trouble with the keyhole opening. I was very cautious to not snip through the stitches, and everything seemed fine, but one of my daughters ripped it when she got pulled. I zigzagged over it, but then one of the others came out of the wash shredded. I saw another topic on this in the forum with a link to a blogpost with a tutorial, but the blog is not active anymore. Any tips on how to fix this on the existing dresses and how to reinforce it as I sew it on future dresses?