I had an SEO and Web Design client do something similar to me once, except she was insisting that she could do all of the work herself. I let her know that while she could alter her website, it would be best if she would let me continue designing and optimising her site (you know, the things she hired me to do) before she made any changes. She ignored my warning and told me that she's more "artistic" than I am, so designing the website and making it appear in the search engine shouldn't be too hard. After she told me this, I knew what was about to happen, so I created a backup of the entire website, grabbed some popcorn, and waited for the panicked phone call.
In case you're wondering what happened to her website, let's just say she discovered that the "design" part was mostly coding and the SEO part had nothing to do with failing to insert text into the code of her website. Luckily for her, all I had to do was restore the files and the website was back to normal.
As for your client, you can either be assertive with the client and explain to him or her that
you are going to do the work, or you can let his daughter sabotage your work and sully your reputation with her possibly subpar SEO skills. Additionally, if you let your client push you around and give them too much control over your work, you're just reinforcing their fanciful notion that they can continue doing it and get away with it.
Communicate with your client; tell them what's on your mind.