Introducing girlfriend - timing question

Been separated 18 months, dating this amazing woman for 6 months. My kids (10M and 7F) have asked about her because they've seen Instagram stuff. I don't want to ambush them but I also don't want to hide her forever. My ex-wife is already dating (found out through the kids) so I don't feel like I'm being unreasonable wanting to have coffee with them and my girlfriend at like a casual spot. Has anyone successfully navigated this without it being weird? What age did you introduce partners?