But you know what has allowed her and I to work so well together? She knows shes not a "parent" to ds. If my x goes to work.. she calls and asks if I want him to come home but also offers to keep him if I have to work etc. If ds comes home from school and asks to participate in something and x isn;t around.. she tells him to call me.
She doesn't necessarily "teach" ds anything. But She does a great job of reinforcing and supporting what his father and I teach him. Yes she is extremely important to ds. He loves her. He makes her pictures at school when he makes them for me too. But both she and ds know that shes not his parent.
She would NEVER try to hide from me what you are proposing to hide from your husbands x. If she did.. it would destroy the great communication line we've forged and we both know it.
She would be far more likely to call and tell me that x had to leave town, but would still like to see ds.. so could he come over the day before x leaves, and then trade for the following weekend.
Good luck with whatever you decide.. but for the sake of the relationship between you and the childrens mother, I'd step back and think before you act on this.