Having been someone who grew up in this sort of household, where one parent goes and another one doesn’t, I know how confusing this can be. I also know how this causes problems in faith development of a child because the child is left wondering why.
Instead of having you answer for the spouse, I think you need to kindly direct your children to your wife for an answer, as only she can answer that. Hiding reasons or making up an excuse only goes so far as kids are smart and will pick up on that behavior and maybe learn that it’s ok to make excuses. I’m in favor for honesty, or better yet shifting the responsibility to the parent who chooses to opt out of Mass to explain why they don’t go. (Being tired and needing sleep may even be a legit reason if one is ill).
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My parent simply put it that they grew up in a different church and left it at that. It wasn't made a big deal only because the children are too young to handle contemplating what this means. It's a sad situation, but hopefully if handled gently and maturely it can turn into a learning/teaching moment.
You also should discuss this matter privately with your spouse beforehand, and make sure not to turn this into something that disrespects your spouse or shuns them, or makes them “the bad guy”. I would highly recommend speaking with your priest about this first as well. I’m sure they’ve had a lot of practice over the years with this scenario.
Im not sure if 4 years old is too young for this, but you can point out to them the scripture where it says a believing spouse should continue to love the unbelieving spouse. I remember being afraid of this matter, when I was old enough to understand something was not jiving with what I was being taught, that my parents would divorce…so be sure to reassure your children you are willing to follow Gods word and continue to love and support your spouse even when they don’t choose something that is otherwise taught as not the thing to do.
Like I said, this can be a teaching moment. Handle it with love, and most definitely pray hard over the matter. God gives us His grace to handle tough moments, but we really need to turn to Him in difficult matters like this. Listen for the Holy Spirits guidance.
Keep in mind too, this isn't too much different from ANY sin being preformed in front of a child to witness, and this world we live in has plenty of bad examples willing to "preach" sin to your kids, so be gentle and loving about this, even if it's angering/frustrating/frightening to you. It's not the end of faith for your children, but it could be if this is handled with raw emotion rather than help of God's grace.
