Set a schedule with the kids. Don't let her constantly reconvene regularly scheduled days to suit her GAL and new single life. They all do this. They make all these plans with the GF's GGW, events concerts, happy hour, wine tasting etc. and expect you to rearrange you're schedule for them because "They planned it in advance" without giving you due notice, and will try and place it on you and sometimes even guilt you into wanting to see your kids so they can go socialize. Just make sure there is an even exchange of reciprocity. Make plans yourself weeks out. See if it conflicts with the days you have the kids, and test the waters to see if she will exchange evenly, like for like if she wants to swap out days. Dont allow her to cake eat on this, and don't enable her for nothing every time she wants to to GALavanting without you.
