The line between acceptable and insulting?

I realize that children don't understand what an insult is. Especially not children under three years of age. But I had to take exception to something that DOM said this evening. As background, we've been trying (mostly unsuccessfully) to potty train her for the past several months (3, 4, 5, 6 months...it all is a giant blur right now). As part of that effort, she has accompanied me to the potty on numerous occasions, so she can see what "big girls" do in the potty. So, today at a restaurant, she came with me to the bathroom, entered the stall, and proceeded to watch me do my business. After a minute of idle chatter, she looked at me, and said, "You have a big bottom, mama!" Gee, thanks kid.