Op is your dd a particularly difficult child? As a parent the instinct is of course to protect our own children, and in our eyes our own children do very little wrong compared to in the eyes of others, ;)
It's also very easy for the mn collective to come on to a thread like this and say "unacceptable for anyone to say anything like that about a four year old child, hang, draw and quarter the woman thrice over given she's the mil."
But the reality is that some children are difficult children, and some people can find a child exasperating without necessarily disliking them. Or might even dislike a child (just because it's a child doesn't mean it has to be likeable) but be able to not show those feelings in front of either the child or the parent. But they're not wrong for expressing feelings to someone else in a private text message.
Mn is full of posts from people who struggle with other people's children and their behaviour The advice is almost always to just smile and not let on, and even distance in some situations.
It doesn't sound as if your mil is treating your dd unfairly. She hasn't said anything to you or your dp. She isn't treating her differently. As hard as it is, she is entitled to her feelings, whatever the reason behind those feelings.
As an aside, why are you reading your mil's text messages?