Be honest. You don’t need to speak for your ex. They will eventually form their own opinion of their mother.
Perhaps something like
Mummy says she can’t visit at the moment. I don’t kow why. You can ask her when you see her. Don’t worry about it. Maybe she will be available soon. And then jump into another positive subject or activity for distraction.
I find the less attention you draw to it the better. They do not realise the importance of what they are asking. If they asked when are we seeing grandad again you would not think twice about saying oh I’m not sure he lives far away but we will arrange something when we can. And then move on in your conversation. ….