How to communicate with ex when she keeps bringing up old stuff?

Every time we text about pickup times or school stuff, my ex brings up something I did wrong during our marriage. I'm trying to keep things focused on the kids but it's exhausting. I use a coparenting app now which helps, but she still manages to slip digs in there. Should I just completely ignore her comments and stay silent, or is there a way to set a boundary without looking like I'm being difficult?