It does sound like a tricky situation. Do you think the strap did come off? If it did, then I think for sure she was owed a refund or a free replacement dress. If it didn't come off, then I would say she's using you. When I first started making bows, I put them on my girls all the time, all day long and let them be rough with the bows, just to make sure I was selling a quality product that would last. So, I would imagine the same is true for clothes, if you have a daughter or a friend with a daughter that you could let her trial your items to make sure they'll hold up to normal wear and tear. Don't get me wrong, I can't sew at all, so I'm not questioning your products, just trying to answer your questions.
I know some boutique sellers post all the measurements of the sizes and encourage customers to measure their children before purchasing and have shop policies that cover such issues, so that'll help cover you if you do decide to start selling them. Good luck!