Custody swap tomorrow and my 7yo is already asking about mom's new boyfriend

So my ex is seeing someone new (found out from my daughter mentioning him casually) and she hasn't told me anything about it. Tomorrow is my weekend and my kids already know this guy is "there" which means they're going to come to me with questions. How do I handle this without seeming like I'm interrogating them or bad-mouthing their mom? My son doesn't care but my daughter is weirdly interested. Should I just answer what she asks or actively not ask questions?