First time introducing the new girlfriend to the kids - tips?

Been dating Sarah for about 8 months now and things are getting serious. My kids are 6 and 11 (so very different maturity levels). I'm planning to take them to get ice cream next weekend and just... have her there casually. Not making a big deal out of it. My ex isn't happy about it obviously but that's not really her call at this point. Anyone else done this? Should I give them a heads up first or keep it casual and low-key? I'm honestly nervous as hell.