I did this with my eldest daughter for the first 2 years of school, taking each Wednesday off. If you tell the school, be prepared to respond to comments like "taking a day off during the week interrupts his/her learning"; "it causes school reluctance", "he/she forgets the rules and finds it harder to resettle"... One of the valid points they may make is that if your child always misses one day of school then they may miss out on art/french/music or whatever lesson is only made available on that day.
I found it caused some confusion among parents/teachers about why I would want to pay for full time school fees and then not want to take advantage of school on a full time basis, etc.
But if you're only thinking about doing this once a month, I would just call it a mental health day. And if you can be flexible about days, then choosing a mental health day spontaneously and as the need arises means that you'll all get the most benefit out of it.
The only thing to consider is that it gets harder and harder to spend quality 1 on 1 time with each child. Why not treat each child to a whole day pursuing one of their own passions - a zoo exploration with one child and a science field trip with another???
Makes for some wonderful memories. I say go for it!
jojo