In this instance because she feels insecure, I would say her.
However, in my own relationship, I don't go round telling my children I love them more than their dad. Their dad is part of them and I don't think it's nice to make comparisons when romantic love and parental love are just so completely different. Once you start saying 'more', why not one child 'more' than the other. Love doesn't divide, it multiplies. I tell my children I love them all the time, I love them to the moon and back or whatever, I don't compare their love to other different forms of love.
My mum never told me she loved me as a child, she was a devoted and tangibly loving parent, and still is- declarations of love on top of that weren't needed. Love is a verb, as people are so often banging on about. So why all this need to downgrade romantic love and declare oneself devoted to one's children.
Weirdly, on the thread about husbands/partners recently, people were declaring that they would only be half a person, totally unable to live, had no meaning without their other halves. I found that equally strange.
Perhaps people just like to declare stuff!