I haven't read your blog posts, but from what you write here I want to say this:
The children's needs come first, and they need to spend time with their dad.
All the adults in a situation can speak for themselves, but you and your husband must speak for your children.
If your metamour is experiencing a crisis of some kind, taking some time away from the kids may be warranted, but it should be clearly understood by all parties as a special circumstance and not something that can be a regular occurrence. Because your children need their father, and there is no substitute for his presence.
If she is just missing him and desperate to be with him, sorry. The kids' needs come first. Besides, she enjoys the lion's share of his time already.
That's my advice for the immediate question at hand. If I may speak to the broader situation, I would strongly caution you that your metamour may be a cowgirl looking to cut your husband out of your herd. She has already stated her desire for a man of her own. And she wants yours. I would watch her actions carefully to determine whether she is moving to cut you out.