How do you handle telling kids about a new partner?

I've been dating someone for 4 months and it's getting serious. My kids are 7 and 9. They know I'm dating but haven't met anyone. My therapist says I should wait longer before introducing them but I feel like 4 months is reasonable? My ex is already saying she'll bring it up to the kids herself if I do it first which feels like a threat. There's nothing in our custody agreement about this. What's the general timeline people use here? I don't want to mess up my relationship with the kids but I also don't want to hide a major part of my life from them forever.